diff --git a/cpp/src/arrow/buffer.cc b/cpp/src/arrow/buffer.cc
index 6d55f88af1e32..2e64ffd75c263 100644
--- a/cpp/src/arrow/buffer.cc
+++ b/cpp/src/arrow/buffer.cc
@@ -92,7 +92,25 @@ Status PoolBuffer::Reserve(int64_t new_capacity) {
 }
 
 Status PoolBuffer::Resize(int64_t new_size) {
-  RETURN_NOT_OK(Reserve(new_size));
+  if (new_size > size_) {
+    RETURN_NOT_OK(Reserve(new_size));
+  } else {
+    // Buffer is not growing, so shrink to the requested size without
+    // excess space.
+    if (capacity_ != new_size) {
+      // Buffer hasn't got yet the requested size.
+      if (new_size == 0) {
+        pool_->Free(mutable_data_, capacity_);
+        capacity_ = 0;
+        mutable_data_ = nullptr;
+        data_ = nullptr;
+      } else {
+        RETURN_NOT_OK(pool_->Reallocate(capacity_, new_size, &mutable_data_));
+        data_ = mutable_data_;
+        capacity_ = new_size;
+      }
+    }
+  }
   size_ = new_size;
   return Status::OK();
 }
diff --git a/cpp/src/arrow/test-util.h b/cpp/src/arrow/test-util.h
index e5957490e0733..f2da824084775 100644
--- a/cpp/src/arrow/test-util.h
+++ b/cpp/src/arrow/test-util.h
@@ -184,7 +184,7 @@ static inline void random_ascii(int n, uint32_t seed, uint8_t* out) {
 
 template <typename T>
 void rand_uniform_int(int n, uint32_t seed, T min_value, T max_value, T* out) {
-  DCHECK(out);
+  DCHECK(out || (n == 0));
   std::mt19937 gen(seed);
   std::uniform_int_distribution<T> d(min_value, max_value);
   for (int i = 0; i < n; ++i) {