diff --git a/ffenv/env.go b/ffenv/env.go
new file mode 100644
index 0000000..6dabb99
--- /dev/null
+++ b/ffenv/env.go
@@ -0,0 +1,89 @@
+package ffenv
+
+import (
+	"bufio"
+	"flag"
+	"fmt"
+	"io"
+	"strings"
+)
+
+// Parser is a parser for .env file format: flag=value. Each
+// line is tokenized as a single key/value pair.
+func Parser(fs *flag.FlagSet) func(io.Reader, func(string, string) error) error {
+	return func(r io.Reader, set func(name, value string) error) error {
+		return parse(fs)("", r, set)
+	}
+}
+
+// Parser is a parser for config files in an extremely simple format. Each
+// line is tokenized as a single key/value pair. The first =-delimited
+// token in the line is interpreted as the flag name, and all remaining tokens
+// are interpreted as the value. Any leading hyphens on the flag name are
+// ignored.
+func parse(fs *flag.FlagSet) func(string, io.Reader, func(string, string) error) error {
+	return func(prefix string, r io.Reader, set func(name, value string) error) error {
+
+		var flags []string
+		fs.VisitAll(func(f *flag.Flag) {
+			flags = append(flags, f.Name)
+		})
+
+		s := bufio.NewScanner(r)
+		for s.Scan() {
+			line := strings.TrimSpace(s.Text())
+			if line == "" {
+				continue // skip empties
+			}
+
+			if line[0] == '#' {
+				continue // skip comments
+			}
+
+			line = strings.TrimPrefix(line, prefix+"_")
+
+			var (
+				name  string
+				value string
+				index = strings.IndexRune(line, '=')
+			)
+			if index < 0 {
+				return fmt.Errorf("wrong format in env file, must be: name=value")
+			}
+
+			name, value = strings.ToLower(line[:index]), line[index+1:]
+
+			if i := strings.Index(value, " #"); i >= 0 {
+				value = strings.TrimSpace(value[:i])
+			}
+
+			var errs []error
+			for _, sep := range []string{"_", "-", ".", "/"} {
+				for _, f := range flags {
+					replaced := strings.ReplaceAll(name, "_", sep)
+					if f == replaced {
+						if err := set(replaced, value); err != nil {
+							errs = append(errs, err)
+							continue
+						}
+						goto done
+					}
+				}
+			}
+			if len(errs) > 0 {
+				return errs[0]
+			}
+		done:
+		}
+		return s.Err()
+	}
+}
+
+// ParserWithPrefix returns a Parser that will remove any prefix on keys in an
+// .env file. For example, given prefix "MY_APP", the line `MY_APP_FOO=bar`
+// in an .env file will be evaluated as name=foo, value=bar.
+func ParserWithPrefix(fs *flag.FlagSet, prefix string) func(io.Reader, func(string, string) error) error {
+	return func(r io.Reader, set func(name, value string) error) error {
+		return parse(fs)(prefix, r, set)
+	}
+}
diff --git a/ffenv/env_test.go b/ffenv/env_test.go
new file mode 100644
index 0000000..a4c18c7
--- /dev/null
+++ b/ffenv/env_test.go
@@ -0,0 +1,285 @@
+package ffenv_test
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/peterbourgon/ff/v3"
+	"github.com/peterbourgon/ff/v3/ffenv"
+	"github.com/peterbourgon/ff/v3/fftest"
+)
+
+type Vars struct {
+	*fftest.Vars
+	S_S     string
+	SDotS   string
+	SDashS  string
+	SSlashS string
+}
+
+func Pair() (*flag.FlagSet, *Vars) {
+	fs, v := fftest.Pair()
+
+	var vv Vars
+	vv.Vars = v
+
+	fs.StringVar(&vv.S_S, "s_s", "", "string")
+	fs.StringVar(&vv.SDashS, "s-s", "", "string")
+	fs.StringVar(&vv.SDotS, "s.s", "", "string")
+	fs.StringVar(&vv.SSlashS, "s/s", "", "string")
+	return fs, &vv
+}
+
+func Compare(want, have *Vars) error {
+	if want.SDashS != have.SDashS {
+		return fmt.Errorf("var SDashS: want %q, have %q", want.SDashS, have.SDashS)
+	}
+	if want.SDotS != have.SDotS {
+		return fmt.Errorf("var SDotS: want %q, have %q", want.SDotS, have.SDotS)
+	}
+	if want.SSlashS != have.SSlashS {
+		return fmt.Errorf("var SSlashS: want %q, have %q", want.SSlashS, have.SSlashS)
+	}
+	if want.S_S != have.S_S {
+		return fmt.Errorf("var S_S: want %q, have %q", want.S_S, have.S_S)
+	}
+	return fftest.Compare(want.Vars, have.Vars)
+}
+
+func TestParseBasics(t *testing.T) {
+	t.Parallel()
+
+	for _, testcase := range []struct {
+		name string
+		env  map[string]string
+		file string
+		args []string
+		opts []ff.Option
+		want Vars
+	}{
+		{
+			name: "empty",
+			args: []string{},
+			want: Vars{Vars: &fftest.Vars{}},
+		},
+		{
+			name: "args only",
+			args: []string{"-s", "foo", "-i", "123", "-b", "-d", "24m"},
+			want: Vars{Vars: &fftest.Vars{S: "foo", I: 123, B: true, D: 24 * time.Minute}},
+		},
+		{
+			name: "file only",
+			file: "testdata/1.env",
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "bar", I: 99, B: true, D: time.Hour}},
+		},
+		{
+			name: "env only",
+			env:  map[string]string{"TEST_PARSE_S": "baz", "TEST_PARSE_F": "0.99", "TEST_PARSE_D": "100s"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "baz", F: 0.99, D: 100 * time.Second}},
+		},
+		{
+			name: "file args",
+			file: "testdata/2.env",
+			args: []string{"-s", "foo", "-i", "1234"},
+			want: Vars{Vars: &fftest.Vars{S: "foo", I: 1234, D: 3 * time.Second}},
+		},
+		{
+			name: "env args",
+			env:  map[string]string{"TEST_PARSE_S": "should be overridden", "TEST_PARSE_B": "true"},
+			args: []string{"-s", "explicit wins", "-i", "7"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "explicit wins", I: 7, B: true}},
+		},
+		{
+			name: "file env",
+			env:  map[string]string{"TEST_PARSE_S": "env takes priority", "TEST_PARSE_B": "true"},
+			file: "testdata/3.env",
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "env takes priority", I: 99, B: true, D: 34 * time.Second}},
+		},
+		{
+			name: "file env args",
+			file: "testdata/4.env",
+			env:  map[string]string{"TEST_PARSE_S": "from env", "TEST_PARSE_I": "300", "TEST_PARSE_F": "0.15", "TEST_PARSE_B": "true"},
+			args: []string{"-s", "from arg", "-i", "100"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "from arg", I: 100, F: 0.15, B: true, D: time.Minute}},
+		},
+		{
+			name: "repeated args",
+			args: []string{"-s", "foo", "-s", "bar", "-d", "1m", "-d", "1h", "-x", "1", "-x", "2", "-x", "3"},
+			want: Vars{Vars: &fftest.Vars{S: "bar", D: time.Hour, X: []string{"1", "2", "3"}}},
+		},
+		{
+			name: "long args",
+			args: []string{"-s_s", "f_oo", "-s-s", "f-oo", "-s.s", "f.oo", "-s/s", "f/oo"},
+			want: Vars{S_S: "f_oo", SDashS: "f-oo", SDotS: "f.oo", SSlashS: "f/oo", Vars: &fftest.Vars{}},
+		},
+		{
+			name: "priority repeats",
+			env:  map[string]string{"TEST_PARSE_S": "s.env", "TEST_PARSE_X": "x.env.1"},
+			file: "testdata/5.env",
+			args: []string{"-s", "s.arg.1", "-s", "s.arg.2", "-x", "x.arg.1", "-x", "x.arg.2"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "s.arg.2", X: []string{"x.arg.1", "x.arg.2"}}}, // highest prio wins and no others are called
+		},
+		{
+			name: "PlainParser string with spaces",
+			file: "testdata/equals.env",
+			want: Vars{Vars: &fftest.Vars{S: "i=am=the=very=model=of=a=modern=major=general"}},
+		},
+		{
+			name: "default comma behavior",
+			env:  map[string]string{"TEST_PARSE_S": "one,two,three", "TEST_PARSE_X": "one,two,three"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{Vars: &fftest.Vars{S: "one,two,three", X: []string{"one,two,three"}}},
+		},
+		{
+			name: "WithEnvVarSplit",
+			env:  map[string]string{"TEST_PARSE_S": "one,two,three", "TEST_PARSE_X": "one,two,three"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE"), ff.WithEnvVarSplit(",")},
+			want: Vars{Vars: &fftest.Vars{S: "three", X: []string{"one", "two", "three"}}},
+		},
+		{
+			name: "WithEnvVarNoPrefix",
+			env:  map[string]string{"TEST_PARSE_S": "foo", "S": "bar"},
+			opts: []ff.Option{ff.WithEnvVarNoPrefix()},
+			want: Vars{Vars: &fftest.Vars{S: "bar"}},
+		},
+		{
+			name: "env var split comma whitespace",
+			env:  map[string]string{"TEST_PARSE_S": "one, two, three ", "TEST_PARSE_X": "one, two, three "},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE"), ff.WithEnvVarSplit(",")},
+			want: Vars{Vars: &fftest.Vars{S: " three ", X: []string{"one", " two", " three "}}},
+		},
+		{
+			name: "flags with . from env",
+			env:  map[string]string{"TEST_PARSE_S_S": "one"},
+			opts: []ff.Option{ff.WithEnvVarPrefix("TEST_PARSE")},
+			want: Vars{S_S: "one", SDashS: "one", SDotS: "one", SSlashS: "one", Vars: &fftest.Vars{}},
+		},
+	} {
+		t.Run(testcase.name, func(t *testing.T) {
+			fs, vars := Pair()
+			if testcase.file != "" {
+				testcase.opts = append(testcase.opts, ff.WithConfigFile(testcase.file), ff.WithConfigFileParser(ffenv.ParserWithPrefix(fs, "TEST_PARSE")))
+			}
+
+			if len(testcase.env) > 0 {
+				for k, v := range testcase.env {
+					defer os.Setenv(k, os.Getenv(k))
+					os.Setenv(k, v)
+				}
+			}
+
+			vars.ParseError = ff.Parse(fs, testcase.args, testcase.opts...)
+			if err := Compare(&testcase.want, vars); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
+func TestParseIssue16(t *testing.T) {
+	t.Parallel()
+
+	for _, testcase := range []struct {
+		name string
+		data string
+		want string
+	}{
+		{
+			name: "hash in value",
+			data: "s=bar#baz",
+			want: "bar#baz",
+		},
+		{
+			name: "EOL comment with space",
+			data: "s=bar # baz",
+			want: "bar",
+		},
+		{
+			name: "EOL comment no space",
+			data: "s=bar #baz",
+			want: "bar",
+		},
+		{
+			name: "only comment with space",
+			data: "#=foo=bar\n",
+			want: "",
+		},
+		{
+			name: "only comment no space",
+			data: "#foo=bar\n",
+			want: "",
+		},
+	} {
+		t.Run(testcase.name, func(t *testing.T) {
+			filename, cleanup := fftest.TempFile(t, testcase.data)
+			defer cleanup()
+
+			fs, vars := fftest.Pair()
+			vars.ParseError = ff.Parse(fs, []string{},
+				ff.WithConfigFile(filename),
+				ff.WithConfigFileParser(ffenv.Parser(fs)),
+			)
+
+			want := fftest.Vars{S: testcase.want}
+			if err := fftest.Compare(&want, vars); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
+func TestParseConfigFile(t *testing.T) {
+	t.Parallel()
+
+	for _, testcase := range []struct {
+		name         string
+		missing      bool
+		allowMissing bool
+		parseError   error
+	}{
+		{
+			name: "has config file",
+		},
+		{
+			name:       "config file missing",
+			missing:    true,
+			parseError: os.ErrNotExist,
+		},
+		{
+			name:         "config file missing + allow missing",
+			missing:      true,
+			allowMissing: true,
+		},
+	} {
+		t.Run(testcase.name, func(t *testing.T) {
+			filename := "dummy"
+			if !testcase.missing {
+				var cleanup func()
+				filename, cleanup = fftest.TempFile(t, "")
+				defer cleanup()
+			}
+
+			fs, vars := fftest.Pair()
+			options := []ff.Option{ff.WithConfigFile(filename), ff.WithConfigFileParser(ffenv.Parser(fs))}
+			if testcase.allowMissing {
+				options = append(options, ff.WithAllowMissingConfigFile(true))
+			}
+
+			vars.ParseError = ff.Parse(fs, []string{}, options...)
+
+			want := fftest.Vars{WantParseErrorIs: testcase.parseError}
+			if err := fftest.Compare(&want, vars); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
diff --git a/ffenv/testdata/1.env b/ffenv/testdata/1.env
new file mode 100644
index 0000000..b0c224a
--- /dev/null
+++ b/ffenv/testdata/1.env
@@ -0,0 +1,4 @@
+TEST_PARSE_S=bar
+TEST_PARSE_I=99
+TEST_PARSE_B=true
+TEST_PARSE_D=1h
diff --git a/ffenv/testdata/2.env b/ffenv/testdata/2.env
new file mode 100644
index 0000000..9d07afc
--- /dev/null
+++ b/ffenv/testdata/2.env
@@ -0,0 +1,4 @@
+
+s=should be overridden
+
+d=3s
diff --git a/ffenv/testdata/3.env b/ffenv/testdata/3.env
new file mode 100644
index 0000000..1b1ecf9
--- /dev/null
+++ b/ffenv/testdata/3.env
@@ -0,0 +1,4 @@
+s=bar
+i=99
+d=34s
+# comment line
diff --git a/ffenv/testdata/4.env b/ffenv/testdata/4.env
new file mode 100644
index 0000000..0b2a9af
--- /dev/null
+++ b/ffenv/testdata/4.env
@@ -0,0 +1,4 @@
+s=from file
+i=200 # comment
+d=1m
+f=2.3
diff --git a/ffenv/testdata/5.env b/ffenv/testdata/5.env
new file mode 100644
index 0000000..bda3679
--- /dev/null
+++ b/ffenv/testdata/5.env
@@ -0,0 +1,5 @@
+s=s.file.1
+s=s.file.2
+
+x=x.file.1
+x=x.file.2
diff --git a/ffenv/testdata/equals.env b/ffenv/testdata/equals.env
new file mode 100644
index 0000000..3a4539d
--- /dev/null
+++ b/ffenv/testdata/equals.env
@@ -0,0 +1 @@
+s=i=am=the=very=model=of=a=modern=major=general
diff --git a/ffenv/testdata/prefix.env b/ffenv/testdata/prefix.env
new file mode 100644
index 0000000..accd25f
--- /dev/null
+++ b/ffenv/testdata/prefix.env
@@ -0,0 +1,5 @@
+TEST_PARSE_S=this
+TEST_PARSE_I=1
+TEST_PARSE_F=1.3
+TEST_PARSE_D=10s
+TEST_PARSE_B=true
diff --git a/ffenv/testdata/solo_bool.env b/ffenv/testdata/solo_bool.env
new file mode 100644
index 0000000..8381d83
--- /dev/null
+++ b/ffenv/testdata/solo_bool.env
@@ -0,0 +1 @@
+s=x
diff --git a/ffenv/testdata/undefined.env b/ffenv/testdata/undefined.env
new file mode 100644
index 0000000..b60db8b
--- /dev/null
+++ b/ffenv/testdata/undefined.env
@@ -0,0 +1,2 @@
+undef=undefined variable
+s=one
diff --git a/testdata/1.conf b/testdata/1.conf
index 48ff474..867b455 100644
--- a/testdata/1.conf
+++ b/testdata/1.conf
@@ -1,4 +1,4 @@
 s bar
 i 99
 b true
-d 1h
\ No newline at end of file
+d 1h