diff --git a/cpp/src/arrow/python/builtin_convert.cc b/cpp/src/arrow/python/builtin_convert.cc
index 9acccc149664..6e59845dea76 100644
--- a/cpp/src/arrow/python/builtin_convert.cc
+++ b/cpp/src/arrow/python/builtin_convert.cc
@@ -390,7 +390,7 @@ class BytesConverter : public TypedConverter<BinaryBuilder> {
       // No error checking
       length = PyBytes_GET_SIZE(bytes_obj);
       bytes = PyBytes_AS_STRING(bytes_obj);
-      RETURN_NOT_OK(typed_builder_->Append(bytes, length));
+      RETURN_NOT_OK(typed_builder_->Append(bytes, static_cast<int32_t>(length)));
     }
     return Status::OK();
   }
@@ -422,7 +422,7 @@ class UTF8Converter : public TypedConverter<StringBuilder> {
       // No error checking
       length = PyBytes_GET_SIZE(bytes_obj);
       bytes = PyBytes_AS_STRING(bytes_obj);
-      RETURN_NOT_OK(typed_builder_->Append(bytes, length));
+      RETURN_NOT_OK(typed_builder_->Append(bytes, static_cast<int32_t>(length)));
     }
     return Status::OK();
   }
diff --git a/cpp/src/arrow/python/common.cc b/cpp/src/arrow/python/common.cc
index a5aea3088446..717cb5c5cc12 100644
--- a/cpp/src/arrow/python/common.cc
+++ b/cpp/src/arrow/python/common.cc
@@ -47,7 +47,7 @@ MemoryPool* get_memory_pool() {
 // ----------------------------------------------------------------------
 // PyBuffer
 
-PyBuffer::PyBuffer(PyObject* obj) : Buffer(nullptr, 0) {
+PyBuffer::PyBuffer(PyObject* obj) : Buffer(nullptr, 0), obj_(nullptr) {
   if (PyObject_CheckBuffer(obj)) {
     obj_ = PyMemoryView_FromObject(obj);
     Py_buffer* buffer = PyMemoryView_GET_BUFFER(obj_);
@@ -61,7 +61,7 @@ PyBuffer::PyBuffer(PyObject* obj) : Buffer(nullptr, 0) {
 
 PyBuffer::~PyBuffer() {
   PyAcquireGIL lock;
-  Py_DECREF(obj_);
+  Py_XDECREF(obj_);
 }
 
 }  // namespace py
diff --git a/cpp/src/arrow/python/pandas-test.cc b/cpp/src/arrow/python/pandas-test.cc
index 0d643df2e9f3..a4e640b83718 100644
--- a/cpp/src/arrow/python/pandas-test.cc
+++ b/cpp/src/arrow/python/pandas-test.cc
@@ -24,20 +24,26 @@
 
 #include "arrow/array.h"
 #include "arrow/builder.h"
-#include "arrow/python/pandas_convert.h"
 #include "arrow/table.h"
 #include "arrow/test-util.h"
 #include "arrow/type.h"
 
+#include "arrow/python/common.h"
+#include "arrow/python/pandas_convert.h"
+
 namespace arrow {
 namespace py {
 
+TEST(PyBuffer, InvalidInputObject) {
+  PyBuffer buffer(Py_None);
+}
+
 TEST(PandasConversionTest, TestObjectBlockWriteFails) {
   StringBuilder builder(default_memory_pool());
   const char value[] = {'\xf1', '\0'};
 
   for (int i = 0; i < 1000; ++i) {
-    builder.Append(value, strlen(value));
+    builder.Append(value, static_cast<int32_t>(strlen(value)));
   }
 
   std::shared_ptr<Array> arr;
diff --git a/cpp/src/arrow/python/pandas_convert.cc b/cpp/src/arrow/python/pandas_convert.cc
index 685b1f421c45..db2e90eb8b0f 100644
--- a/cpp/src/arrow/python/pandas_convert.cc
+++ b/cpp/src/arrow/python/pandas_convert.cc
@@ -159,13 +159,13 @@ Status AppendObjectStrings(StringBuilder& string_builder, PyObject** objects,
         PyErr_Clear();
         return Status::TypeError("failed converting unicode to UTF8");
       }
-      const int64_t length = PyBytes_GET_SIZE(obj);
+      const int32_t length = static_cast<int32_t>(PyBytes_GET_SIZE(obj));
       Status s = string_builder.Append(PyBytes_AS_STRING(obj), length);
       Py_DECREF(obj);
       if (!s.ok()) { return s; }
     } else if (PyBytes_Check(obj)) {
       *have_bytes = true;
-      const int64_t length = PyBytes_GET_SIZE(obj);
+      const int32_t length = static_cast<int32_t>(PyBytes_GET_SIZE(obj));
       RETURN_NOT_OK(string_builder.Append(PyBytes_AS_STRING(obj), length));
     } else {
       string_builder.AppendNull();
@@ -235,7 +235,7 @@ class PandasConverter : public TypeVisitor {
   }
 
   Status InitNullBitmap() {
-    int null_bytes = BitUtil::BytesForBits(length_);
+    int64_t null_bytes = BitUtil::BytesForBits(length_);
 
     null_bitmap_ = std::make_shared<PoolBuffer>(pool_);
     RETURN_NOT_OK(null_bitmap_->Resize(null_bytes));
@@ -357,7 +357,7 @@ inline Status PandasConverter::ConvertData(std::shared_ptr<Buffer>* data) {
 
 template <>
 inline Status PandasConverter::ConvertData<BooleanType>(std::shared_ptr<Buffer>* data) {
-  int nbytes = BitUtil::BytesForBits(length_);
+  int64_t nbytes = BitUtil::BytesForBits(length_);
   auto buffer = std::make_shared<PoolBuffer>(pool_);
   RETURN_NOT_OK(buffer->Resize(nbytes));
 
@@ -423,7 +423,7 @@ Status PandasConverter::ConvertBooleans(std::shared_ptr<Array>* out) {
 
   PyObject** objects = reinterpret_cast<PyObject**>(PyArray_DATA(arr_));
 
-  int nbytes = BitUtil::BytesForBits(length_);
+  int64_t nbytes = BitUtil::BytesForBits(length_);
   auto data = std::make_shared<PoolBuffer>(pool_);
   RETURN_NOT_OK(data->Resize(nbytes));
   uint8_t* bitmap = data->mutable_data();
diff --git a/cpp/src/arrow/python/pandas_convert.h b/cpp/src/arrow/python/pandas_convert.h
index a33741efaa49..12644d98da15 100644
--- a/cpp/src/arrow/python/pandas_convert.h
+++ b/cpp/src/arrow/python/pandas_convert.h
@@ -31,7 +31,7 @@ namespace arrow {
 
 class Array;
 class Column;
-class DataType;
+struct DataType;
 class MemoryPool;
 class Status;
 class Table;