diff --git a/lib/rb/spec/ThriftSpec.thrift b/lib/rb/spec/ThriftSpec.thrift
index 3c2f3e0a87..a83014d97c 100644
--- a/lib/rb/spec/ThriftSpec.thrift
+++ b/lib/rb/spec/ThriftSpec.thrift
@@ -185,3 +185,19 @@ struct NestedMapInMapKey {
 struct NestedMapInMapValue {
   2: map<byte, map<byte, byte>> value
 }
+
+# Recursive types for the struct/union/exception recursion-depth limit specs (THRIFT-6045).
+struct RecTree {
+  1: list<RecTree> children
+  2: i16 item
+}
+
+union RecUnion {
+  1: list<RecUnion> children
+  2: i32 leaf
+}
+
+exception RecError {
+  1: list<RecError> children
+  2: i32 leaf
+}
diff --git a/lib/rb/spec/recursion_depth_spec.rb b/lib/rb/spec/recursion_depth_spec.rb
new file mode 100644
index 0000000000..5ccb9091b7
--- /dev/null
+++ b/lib/rb/spec/recursion_depth_spec.rb
@@ -0,0 +1,223 @@
+# frozen_string_literal: true
+#
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements. See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership. The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License. You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied. See the License for the
+# specific language governing permissions and limitations
+# under the License.
+#
+
+require 'spec_helper'
+
+# Round-trip test for struct/union/exception recursion depth, driving the
+# *generated* read/write path (Thrift::Struct#read/#write,
+# Thrift::Union#read/#write) over RecTree / RecUnion / RecError from
+# ThriftSpec.thrift; a linear chain is one struct level deeper per node, so a
+# chain of N nodes reaches depth N.
+#
+# NOTE: the Ruby library does not enforce a recursion-depth limit yet
+# (THRIFT-6045). The round-trip / within-limit examples are active; the
+# limit-enforcement (over-limit) examples are `pending` and will start passing
+# once the limit is implemented, at which point RSpec flags them to be enabled.
+describe 'recursion depth limit' do
+  # The intended struct/union nesting limit. The Ruby library does not enforce a
+  # recursion-depth limit yet (THRIFT-6045); the limit-enforcement examples below
+  # are therefore `pending` until it is implemented. 64 matches the limit other
+  # Thrift libraries use.
+  RECURSION_LIMIT = 64
+
+  # Attached to the pending (over-limit) examples.
+  PENDING_REASON = 'recursion-depth limit not implemented in the Ruby library yet (THRIFT-6045)'
+
+  def binary_protocol
+    Thrift::BinaryProtocol.new(Thrift::MemoryBufferTransport.new)
+  end
+
+  # A linearly nested RecTree that is `depth` struct levels deep.
+  def struct_chain(depth)
+    node = SpecNamespace::RecTree.new(item: depth, children: [])
+    node.children = [struct_chain(depth - 1)] if depth > 1
+    node
+  end
+
+  def tree_depth(node)
+    n = 0
+    until node.nil?
+      n += 1
+      break if node.children.nil? || node.children.empty?
+      node = node.children.first
+    end
+    n
+  end
+
+  # A linearly nested RecUnion that is `depth` levels deep (each union holds the
+  # next; the innermost holds a scalar leaf).
+  def union_chain(depth)
+    if depth > 1
+      SpecNamespace::RecUnion.new(children: [union_chain(depth - 1)])
+    else
+      SpecNamespace::RecUnion.new(leaf: 0)
+    end
+  end
+
+  # A linearly nested RecError exception that is `depth` struct levels deep.
+  # Exceptions read/write through the same generated path as structs, so
+  # tree_depth applies to the decoded chain too.
+  def error_chain(depth)
+    node = SpecNamespace::RecError.new(leaf: depth, children: [])
+    node.children = [error_chain(depth - 1)] if depth > 1
+    node
+  end
+
+  # Emit, via raw protocol calls (which carry no depth guard), the wire image of
+  # a RecTree chain `depth` levels deep. This lets the read tests feed an
+  # over-limit payload that the guarded writer would itself refuse to produce.
+  def write_raw_tree(oprot, depth)
+    oprot.write_struct_begin('RecTree')
+    oprot.write_field_begin('children', Thrift::Types::LIST, 1)
+    oprot.write_list_begin(Thrift::Types::STRUCT, depth > 1 ? 1 : 0)
+    write_raw_tree(oprot, depth - 1) if depth > 1
+    oprot.write_list_end
+    oprot.write_field_end
+    oprot.write_field_begin('item', Thrift::Types::I16, 2)
+    oprot.write_i16(depth)
+    oprot.write_field_end
+    oprot.write_field_stop
+    oprot.write_struct_end
+  end
+
+  # Same as write_raw_tree but for the RecError exception (leaf is i32 here).
+  def write_raw_error(oprot, depth)
+    oprot.write_struct_begin('RecError')
+    oprot.write_field_begin('children', Thrift::Types::LIST, 1)
+    oprot.write_list_begin(Thrift::Types::STRUCT, depth > 1 ? 1 : 0)
+    write_raw_error(oprot, depth - 1) if depth > 1
+    oprot.write_list_end
+    oprot.write_field_end
+    oprot.write_field_begin('leaf', Thrift::Types::I32, 2)
+    oprot.write_i32(depth)
+    oprot.write_field_end
+    oprot.write_field_stop
+    oprot.write_struct_end
+  end
+
+  def expect_depth_limit
+    expect { yield }.to raise_error(Thrift::ProtocolException) { |e|
+      expect(e.type).to eq(Thrift::ProtocolException::DEPTH_LIMIT)
+    }
+  end
+
+  describe 'structs' do
+    it 'round-trips a chain at the limit' do
+      prot = binary_protocol
+      struct_chain(RECURSION_LIMIT).write(prot)
+      result = SpecNamespace::RecTree.new
+      result.read(prot)
+      expect(tree_depth(result)).to eq(RECURSION_LIMIT)
+    end
+
+    it 'rejects writing a chain past the limit' do
+      pending PENDING_REASON
+      expect_depth_limit { struct_chain(RECURSION_LIMIT + 1).write(binary_protocol) }
+    end
+
+    it 'rejects reading a payload past the limit' do
+      pending PENDING_REASON
+      prot = binary_protocol
+      write_raw_tree(prot, RECURSION_LIMIT + 1)
+      expect_depth_limit { SpecNamespace::RecTree.new.read(prot) }
+    end
+
+    it 'round-trips a wide shallow tree (counter unwinds per sibling)' do
+      width = RECURSION_LIMIT * 3
+      prot = binary_protocol
+      root = SpecNamespace::RecTree.new(
+        item: 0,
+        children: (1..width).map { |i| SpecNamespace::RecTree.new(item: i, children: []) }
+      )
+      root.write(prot)
+      result = SpecNamespace::RecTree.new
+      result.read(prot)
+      expect(result.children.size).to eq(width)
+    end
+  end
+
+  describe 'unions' do
+    it 'round-trips a chain at the limit' do
+      prot = binary_protocol
+      union_chain(RECURSION_LIMIT).write(prot)
+      expect { SpecNamespace::RecUnion.new.read(prot) }.not_to raise_error
+    end
+
+    it 'rejects writing a chain past the limit' do
+      pending PENDING_REASON
+      expect_depth_limit { union_chain(RECURSION_LIMIT + 1).write(binary_protocol) }
+    end
+  end
+
+  describe 'exceptions' do
+    it 'round-trips a chain at the limit' do
+      prot = binary_protocol
+      error_chain(RECURSION_LIMIT).write(prot)
+      result = SpecNamespace::RecError.new
+      result.read(prot)
+      expect(tree_depth(result)).to eq(RECURSION_LIMIT)
+    end
+
+    it 'rejects writing a chain past the limit' do
+      pending PENDING_REASON
+      expect_depth_limit { error_chain(RECURSION_LIMIT + 1).write(binary_protocol) }
+    end
+
+    it 'rejects reading a payload past the limit' do
+      pending PENDING_REASON
+      prot = binary_protocol
+      write_raw_error(prot, RECURSION_LIMIT + 1)
+      expect_depth_limit { SpecNamespace::RecError.new.read(prot) }
+    end
+  end
+
+  describe 'protocol decorators' do
+    # A struct written through a decorator must still be bounded and must not
+    # crash: decorators (MultiplexedProtocol via ProtocolDecorator) do not chain
+    # BaseProtocol#initialize, so the depth counter starts unset on that object.
+    it 'round-trips a struct through a MultiplexedProtocol' do
+      mprot = Thrift::MultiplexedProtocol.new(binary_protocol, 'svc')
+      struct_chain(3).write(mprot)
+      result = SpecNamespace::RecTree.new
+      result.read(mprot)
+      expect(tree_depth(result)).to eq(3)
+    end
+  end
+
+  # Only present when the native (thrift_native) extension is loaded, which
+  # also makes Thrift::Struct#read/#write the native implementations -- the path
+  # that must enforce the limit in C, not just in pure Ruby.
+  if defined? Thrift::BinaryProtocolAccelerated
+    describe 'accelerated binary protocol' do
+      it 'rejects writing a chain past the limit' do
+        pending PENDING_REASON
+        prot = Thrift::BinaryProtocolAccelerated.new(Thrift::MemoryBufferTransport.new)
+        expect_depth_limit { struct_chain(RECURSION_LIMIT + 1).write(prot) }
+      end
+
+      it 'rejects reading a payload past the limit' do
+        pending PENDING_REASON
+        prot = Thrift::BinaryProtocolAccelerated.new(Thrift::MemoryBufferTransport.new)
+        write_raw_tree(prot, RECURSION_LIMIT + 1)
+        expect_depth_limit { SpecNamespace::RecTree.new.read(prot) }
+      end
+    end
+  end
+end