diff --git a/lib/net/http.rb b/lib/net/http.rb
index 6c43e62..5ab4cf9 100644
--- a/lib/net/http.rb
+++ b/lib/net/http.rb
@@ -688,6 +688,9 @@ class HTTPHeaderSyntaxError < StandardError; end
   #   Returns the flags for server the certification verification at the beginning of the SSL/TLS session.
   # - {:verify_mode=}[rdoc-ref:Net::HTTP#verify_mode=]:
   #   Sets the flags for server the certification verification at the beginning of the SSL/TLS session.
+  # - {#set_debug_output}[rdoc-ref:Net::HTTP#set_debug_output]:
+  #   Sets the output stream for debugging, which logs credentials in plain text;
+  #   never use it in production code.
   #
   # === Addresses and Ports
   #
@@ -1260,8 +1263,22 @@ def inspect
     #   read 2 bytes
     #   Conn keep-alive
     #
+    # Debug output contains credentials, so enabling it writes a warning to
+    # <tt>$stderr</tt>. The warning is written at most once per thread, and is
+    # not written when <tt>$DEBUG</tt> is set, which already declares that the
+    # process is being debugged, or when +output+ is +nil+, which disables debug
+    # output. Like any other Ruby warning it can be intercepted by overriding
+    # Warning.warn.
+    #
     def set_debug_output(output)
       warn 'Net::HTTP#set_debug_output called after HTTP started', uplevel: 1 if started?
+      if output and !$DEBUG and !Thread.current.thread_variable_get(:net_http_debug_output_warned)
+        Thread.current.thread_variable_set(:net_http_debug_output_warned, true)
+        warn 'Net::HTTP#set_debug_output: every request and response, including ' +
+          'Authorization and Cookie headers and message bodies, will be written ' +
+          'to the given stream in plain text; never enable this in production',
+          uplevel: 1
+      end
       @debug_output = output
     end
 
diff --git a/test/net/http/test_http.rb b/test/net/http/test_http.rb
index e5028d4..a1bd6a8 100644
--- a/test/net/http/test_http.rb
+++ b/test/net/http/test_http.rb
@@ -7,6 +7,10 @@
 
 class TestNetHTTP < Test::Unit::TestCase
 
+  def setup
+    Thread.current.thread_variable_set(:net_http_debug_output_warned, nil)
+  end
+
   def test_class_Proxy
     no_proxy_class = Net::HTTP.Proxy nil
 
@@ -266,6 +270,76 @@ def test_default_configuration
     Net::HTTP.default_configuration = nil
   end
 
+  def test_set_debug_output_warns
+    http = Net::HTTP.new 'hostname.example'
+
+    assert_warning(/#{__FILE__}:#{__LINE__+1}: warning: Net::HTTP#set_debug_output: /) do
+      http.set_debug_output StringIO.new
+    end
+  end
+
+  def test_set_debug_output_names_what_is_logged
+    http = Net::HTTP.new 'hostname.example'
+
+    assert_warning(/Authorization and Cookie headers and message bodies/) do
+      http.set_debug_output StringIO.new
+    end
+  end
+
+  def test_set_debug_output_nil_does_not_warn
+    http = Net::HTTP.new 'hostname.example'
+
+    assert_warning('') do
+      http.set_debug_output nil
+    end
+  end
+
+  def test_set_debug_output_DEBUG_does_not_warn
+    debug, $DEBUG = $DEBUG, true
+
+    http = Net::HTTP.new 'hostname.example'
+
+    assert_warning('') do
+      http.set_debug_output StringIO.new
+    end
+  ensure
+    $DEBUG = debug
+  end
+
+  def test_set_debug_output_warns_once_per_thread
+    http = Net::HTTP.new 'hostname.example'
+
+    assert_warning(/Net::HTTP#set_debug_output: /) do
+      http.set_debug_output StringIO.new
+    end
+
+    assert_warning('') do
+      http.set_debug_output StringIO.new
+    end
+  end
+
+  def test_set_debug_output_warns_once_across_objects
+    assert_warning(/Net::HTTP#set_debug_output: /) do
+      Net::HTTP.new('hostname.example').set_debug_output StringIO.new
+    end
+
+    assert_warning('') do
+      Net::HTTP.new('hostname.example').set_debug_output StringIO.new
+    end
+  end
+
+  def test_set_debug_output_warns_once_in_subclasses
+    assert_warning(/Net::HTTP#set_debug_output: /) do
+      Net::HTTP.new('hostname.example').set_debug_output StringIO.new
+    end
+
+    proxy = Net::HTTP::Proxy('proxy.example', 8000)
+
+    assert_warning('') do
+      proxy.new('hostname.example').set_debug_output StringIO.new
+    end
+  end
+
 end
 
 module TestNetHTTP_version_1_1_methods
@@ -1470,4 +1544,18 @@ def test_get
       assert_equal expected, ret
     RUBY
   end
+
+  # The once-per-thread flag is kept in thread-local storage rather than on
+  # Net::HTTP, because non-main Ractors cannot set instance variables of
+  # classes. Recording it there would raise Ractor::IsolationError.
+  def test_set_debug_output
+    assert_ractor(<<~RUBY, require: 'net/http', ignore_stderr: true)
+      require 'stringio'
+      ret = Ractor.new {
+        Net::HTTP.new('hostname.example').set_debug_output(StringIO.new)
+        :ok
+      }.value
+      assert_equal :ok, ret
+    RUBY
+  end
 end if defined?(Ractor) && Ractor.method_defined?(:value)