diff --git a/src/foundation.jl b/src/foundation.jl
index 6e01143..d100f72 100644
--- a/src/foundation.jl
+++ b/src/foundation.jl
@@ -92,6 +92,56 @@ NSValue(x::Union{NSRange,UnitRange}) =
 # ...
 
 
+export NSDecimal
+
+const NSDecimalMaxSize = 8
+struct NSDecimal
+    # an integer from –128 through 127
+    exponent::Int8
+
+    # unsigned int _length:4;
+    #   length == 0 && isNegative -> NaN
+    # unsigned int _isNegative:1;
+    # unsigned int _isCompact:1;
+    # unsigned int _reserved:18
+    flags::UInt8
+    _reserved1::UInt8
+    _reserved2::UInt8
+
+    # a decimal integer up to 38 digits long
+    mantissa::NTuple{NSDecimalMaxSize, Cushort}
+end
+
+function Base.getproperty(obj::NSDecimal, sym::Symbol)
+    if sym == :length
+        obj.flags & 0xf
+    elseif sym == :isNegative
+        Bool(obj.flags >> 4 & 1)
+    elseif sym == :isCompact
+        obj.flags >> 5 & 1
+    elseif sym == :mantissa
+        bits = getfield(obj, :mantissa)
+        result = UInt128(0)
+        for i in 1:obj.length
+            result |= UInt128(bits[i]) << (16*(i-1))
+        end
+        result
+    else
+        getfield(obj, sym)
+    end
+end
+
+Base.isnan(dec::NSDecimal) = dec.length == 0 && dec.isNegative
+
+function Base.show(io::IO, dec::NSDecimal)
+  if isnan(dec)
+    print(io, "NaN")
+  else
+    print(io, "NSDecimal(", dec.mantissa, "e", dec.exponent, ")")
+  end
+end
+
+
 export NSNumber
 
 @objcwrapper NSNumber <: NSObject
@@ -99,7 +149,7 @@ export NSNumber
 @objcproperties NSNumber begin
     @autoproperty boolValue::Bool
     @autoproperty charValue::Cchar
-    #@autoproperty decimalValue::id{NSDecimalNumber}
+    @autoproperty decimalValue::NSDecimal
     @autoproperty doubleValue::Cdouble
     @autoproperty floatValue::Cfloat
     @autoproperty intValue::Cint
@@ -143,6 +193,57 @@ let
 end
 
 
+export NSDecimalNumber, NaNDecimalNumber
+
+@objcwrapper NSDecimalNumber <: NSNumber
+@objcproperties NSDecimalNumber begin
+    #@autoproperty defaultBehavior::NSDecimalNumberBehaviors
+    @autoproperty decimalValue::NSDecimal
+    @autoproperty doubleValue::Float64
+end
+
+# constructors
+NSDecimalNumber(dec::NSDecimal) =
+    NSDecimalNumber(@objc [NSDecimalNumber decimalNumberWithDecimal:dec::NSDecimal]::id{NSDecimalNumber})
+NSDecimalNumber(str::Union{String,NSString}) =
+    NSDecimalNumber(@objc [NSDecimalNumber decimalNumberWithString:str::id{NSString}]::id{NSDecimalNumber})
+NSDecimalNumber(; mantissa, exponent, negative) =
+    NSDecimalNumber(@objc [NSDecimalNumber decimalNumberWithMantissa:mantissa::Culonglong
+                                                            exponent:exponent::Cshort
+                                                          isNegative:negative::Bool]::id{NSDecimalNumber})
+
+# conversions
+Float64(dec::NSDecimal) = NSDecimalNumber(dec).doubleValue
+NSDecimal(dec::NSDecimalNumber) = dec.decimalValue
+
+# fixed values
+Base.zero(::Type{NSDecimalNumber}) =
+    NSDecimalNumber(@objc [NSDecimalNumber zero]::id{NSDecimalNumber})
+Base.one(::Type{NSDecimalNumber}) =
+    NSDecimalNumber(@objc [NSDecimalNumber one]::id{NSDecimalNumber})
+Base.typemin(::Type{NSDecimalNumber}) =
+    NSDecimalNumber(@objc [NSDecimalNumber minimumDecimalNumber]::id{NSDecimalNumber})
+Base.typemax(::Type{NSDecimalNumber}) =
+    NSDecimalNumber(@objc [NSDecimalNumber maximumDecimalNumber]::id{NSDecimalNumber})
+NaNDecimalNumber() =
+    NSDecimalNumber(@objc [NSDecimalNumber notANumber]::id{NSDecimalNumber})
+
+# output
+Base.string(number::NSDecimalNumber) = number.description
+Base.show(io::IO, number::NSDecimalNumber) = print(io, "NSDecimalNumber(", number.description, ")")
+
+# comparisons
+@cenum NSComparisonResult::NSInteger begin
+    NSOrderedAscending = -1
+    NSOrderedSame = 0
+    NSOrderedDescending = 1
+end
+compare(a::NSDecimalNumber, b::NSDecimalNumber) =
+    @objc [a::id{NSDecimalNumber} compare:b::id{NSDecimalNumber}]::NSComparisonResult
+Base.isequal(a::NSDecimalNumber, b::NSDecimalNumber) = compare(a, b) == NSOrderedSame
+Base.isless(a::NSDecimalNumber, b::NSDecimalNumber) = compare(a, b) == NSOrderedAscending
+
+
 export NSString
 
 #@objcwrapper NSString <: NSObject
diff --git a/test/runtests.jl b/test/runtests.jl
index b50f011..09d04d5 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -239,6 +239,34 @@ end
         @test convert(NSNumber, 123) == i
         @test convert(NSNumber, 456) == j
     end
+
+    @testset "decimal" begin
+        i = NSNumber(3.14)
+        @test i.decimalValue == NSDecimal(NSDecimalNumber("3.14"))
+    end
+end
+
+@testset "NSDecimal" begin
+    @test zero(NSDecimalNumber) == zero(NSDecimalNumber)
+    @test zero(NSDecimalNumber) != one(NSDecimalNumber)
+    @test zero(NSDecimalNumber) < one(NSDecimalNumber)
+    @test one(NSDecimalNumber) > zero(NSDecimalNumber)
+    @test one(NSDecimalNumber) != NaNDecimalNumber()
+    @test one(NSDecimalNumber) > typemin(NSDecimalNumber)
+    @test one(NSDecimalNumber) < typemax(NSDecimalNumber)
+
+    # –12.345, expressed as 12345x10^–3: mantissa 12345; exponent –3; isNegative YES
+    num = NSDecimalNumber(mantissa=12345, exponent=-3, negative=true)
+    @test num == NSDecimalNumber("-12.345")
+
+    # conversion to raw NSDecimal
+    dec = NSDecimal(num)
+    @test dec.mantissa == 12345
+    @test dec.exponent == -3
+    @test dec.isNegative
+
+    # conversion back to NSDecimalNumber
+    @test NSDecimalNumber(dec) == num
 end
 
 @testset "NSURL" begin