diff --git a/cpp/src/arrow/io/buffered.cc b/cpp/src/arrow/io/buffered.cc
index b41e4257af215..25cf79c78d916 100644
--- a/cpp/src/arrow/io/buffered.cc
+++ b/cpp/src/arrow/io/buffered.cc
@@ -51,6 +51,7 @@ class BufferedBase {
     return !is_open_;
   }
 
+  // Allocate buffer_ if needed, and resize it to buffer_size_ if required.
   Status ResetBuffer() {
     if (!buffer_) {
       // On first invocation, or if the buffer has been released, we allocate a
@@ -283,18 +284,32 @@ class BufferedInputStream::Impl : public BufferedBase {
   }
 
   // Resize internal read buffer. Note that the internal buffer-size
-  // should be not larger than the raw_read_bound_.
+  // should not be larger than the raw_read_bound_.
+  // It might change the buffer_size_, but will not change buffer states
+  // buffer_pos_ and bytes_buffered_.
   Status SetBufferSize(int64_t new_buffer_size) {
     if (new_buffer_size <= 0) {
       return Status::Invalid("Buffer size should be positive");
     }
     if ((buffer_pos_ + bytes_buffered_) >= new_buffer_size) {
-      return Status::Invalid("Cannot shrink read buffer if buffered data remains");
+      return Status::Invalid(
+          "Cannot shrink read buffer if buffered data remains, new_buffer_size: ",
+          new_buffer_size, ", buffer_pos: ", buffer_pos_,
+          ", bytes_buffered: ", bytes_buffered_, ", buffer_size: ", buffer_size_);
     }
     if (raw_read_bound_ >= 0) {
       // No need to reserve space for more than the total remaining number of bytes.
-      new_buffer_size = std::min(new_buffer_size,
-                                 bytes_buffered_ + (raw_read_bound_ - raw_read_total_));
+      if (bytes_buffered_ == 0) {
+        // Special case: we can not keep the current buffer because it does not
+        // contain any required data.
+        new_buffer_size = std::min(new_buffer_size, raw_read_bound_ - raw_read_total_);
+      } else {
+        // We should keep the current buffer because it contains data that
+        // can be read.
+        new_buffer_size =
+            std::min(new_buffer_size,
+                     buffer_pos_ + bytes_buffered_ + (raw_read_bound_ - raw_read_total_));
+      }
     }
     return ResizeBuffer(new_buffer_size);
   }
@@ -350,7 +365,7 @@ class BufferedInputStream::Impl : public BufferedBase {
   }
 
   Status DoBuffer() {
-    // Fill buffer
+    // Fill the buffer from the raw stream with at most `buffer_size_` bytes.
     if (!buffer_) {
       RETURN_NOT_OK(ResetBuffer());
     }
@@ -444,8 +459,8 @@ class BufferedInputStream::Impl : public BufferedBase {
   // The default -1 indicates that it is unbounded
   int64_t raw_read_bound_;
 
-  // Number of remaining bytes in the buffer, to be reduced on each read from
-  // the buffer
+  // Number of remaining valid bytes in the buffer, to be reduced on each read
+  // from the buffer.
   int64_t bytes_buffered_;
 };
 
diff --git a/cpp/src/arrow/io/buffered.h b/cpp/src/arrow/io/buffered.h
index 01c0a016daba0..22ea7520a5050 100644
--- a/cpp/src/arrow/io/buffered.h
+++ b/cpp/src/arrow/io/buffered.h
@@ -111,6 +111,7 @@ class ARROW_EXPORT BufferedInputStream
       int64_t raw_read_bound = -1);
 
   /// \brief Resize internal read buffer; calls to Read(...) will read at least
+  ///        this many bytes from the raw InputStream if possible.
   /// \param[in] new_buffer_size the new read buffer size
   /// \return Status
   Status SetBufferSize(int64_t new_buffer_size);
diff --git a/cpp/src/arrow/io/buffered_test.cc b/cpp/src/arrow/io/buffered_test.cc
index 89fe4b159f341..1d4805f580c39 100644
--- a/cpp/src/arrow/io/buffered_test.cc
+++ b/cpp/src/arrow/io/buffered_test.cc
@@ -491,6 +491,29 @@ TEST_F(TestBufferedInputStream, BufferSizeLimit) {
   }
 }
 
+TEST_F(TestBufferedInputStream, PeekPastBufferedBytes) {
+  // GH-43949: Peek and SetBufferSize should not affect the
+  // buffered bytes.
+  MakeExample1(/*buffer_size=*/10, default_memory_pool(), /*raw_read_bound=*/15);
+  ASSERT_OK_AND_ASSIGN(auto bytes, buffered_->Read(9));
+  EXPECT_EQ(std::string_view(*bytes), kExample1.substr(0, 9));
+  ASSERT_EQ(1, buffered_->bytes_buffered());
+  ASSERT_EQ(10, buffered_->buffer_size());
+  ASSERT_OK_AND_ASSIGN(auto view, buffered_->Peek(3));
+  EXPECT_EQ(view, kExample1.substr(9, 3));
+  ASSERT_EQ(3, buffered_->bytes_buffered());
+  ASSERT_EQ(12, buffered_->buffer_size());
+  ASSERT_OK_AND_ASSIGN(view, buffered_->Peek(10));
+  // Peek() cannot go past the `raw_read_bound`
+  EXPECT_EQ(view, kExample1.substr(9, 6));
+  ASSERT_EQ(6, buffered_->bytes_buffered());
+  ASSERT_EQ(15, buffered_->buffer_size());
+  // Do read
+  ASSERT_OK_AND_ASSIGN(bytes, buffered_->Read(6));
+  EXPECT_EQ(std::string_view(*bytes), kExample1.substr(9, 6));
+  ASSERT_EQ(0, buffered_->bytes_buffered());
+}
+
 class TestBufferedInputStreamBound : public ::testing::Test {
  public:
   void SetUp() { CreateExample(/*bounded=*/true); }