diff --git a/i18n/data/en.po b/i18n/data/en.po
index 50dab89fbc4..4855ee35469 100644
--- a/i18n/data/en.po
+++ b/i18n/data/en.po
@@ -395,7 +395,7 @@ msgstr "Compiling core..."
 msgid "Compiling libraries..."
 msgstr "Compiling libraries..."
 
-#: legacy/builder/phases/libraries_builder.go:135
+#: legacy/builder/phases/libraries_builder.go:136
 msgid "Compiling library \"{0}\""
 msgstr "Compiling library \"{0}\""
 
@@ -1358,7 +1358,7 @@ msgstr "Library installed"
 msgid "Library name"
 msgstr "Library name"
 
-#: legacy/builder/phases/libraries_builder.go:91
+#: legacy/builder/phases/libraries_builder.go:92
 msgid "Library {0} has been declared precompiled:"
 msgstr "Library {0} has been declared precompiled:"
 
@@ -1745,8 +1745,8 @@ msgstr "Port closed:"
 msgid "Port monitor error"
 msgstr "Port monitor error"
 
-#: legacy/builder/phases/libraries_builder.go:101
-#: legacy/builder/phases/libraries_builder.go:109
+#: legacy/builder/phases/libraries_builder.go:102
+#: legacy/builder/phases/libraries_builder.go:110
 msgid "Precompiled library in \"{0}\" not found"
 msgstr "Precompiled library in \"{0}\" not found"
 
@@ -2069,7 +2069,7 @@ msgstr "The key '%[1]v' is not a list of items, can't remove from it.\n"
 msgid "The output format for the logs, can be: %s"
 msgstr "The output format for the logs, can be: %s"
 
-#: legacy/builder/phases/libraries_builder.go:151
+#: legacy/builder/phases/libraries_builder.go:152
 msgid "The platform does not support '{0}' for precompiled libraries."
 msgstr "The platform does not support '{0}' for precompiled libraries."
 
@@ -2333,8 +2333,8 @@ msgstr "Using library {0} in folder: {1} {2}"
 msgid "Using precompiled core: {0}"
 msgstr "Using precompiled core: {0}"
 
-#: legacy/builder/phases/libraries_builder.go:98
-#: legacy/builder/phases/libraries_builder.go:106
+#: legacy/builder/phases/libraries_builder.go:99
+#: legacy/builder/phases/libraries_builder.go:107
 msgid "Using precompiled library in {0}"
 msgstr "Using precompiled library in {0}"
 
diff --git a/legacy/builder/phases/libraries_builder.go b/legacy/builder/phases/libraries_builder.go
index a5253f76303..a3d107d002b 100644
--- a/legacy/builder/phases/libraries_builder.go
+++ b/legacy/builder/phases/libraries_builder.go
@@ -37,7 +37,8 @@ type LibrariesBuilder struct{}
 func (s *LibrariesBuilder) Run(ctx *types.Context) error {
 	librariesBuildPath := ctx.LibrariesBuildPath
 	buildProperties := ctx.BuildProperties
-	includes := utils.Map(ctx.IncludeFolders.AsStrings(), utils.WrapWithHyphenI)
+    libFolders := append(ctx.IncludeFolders.AsStrings(), ctx.BuildPath.Join("sketch").Join("libdefs").String()) // user definitions for library include files
+    includes := utils.Map(libFolders, utils.WrapWithHyphenI)
 	libs := ctx.ImportedLibraries
 
 	if err := librariesBuildPath.MkdirAll(); err != nil {
diff --git a/legacy/builder/phases/sketch_builder.go b/legacy/builder/phases/sketch_builder.go
index 8fea0f129c0..3ad391228db 100644
--- a/legacy/builder/phases/sketch_builder.go
+++ b/legacy/builder/phases/sketch_builder.go
@@ -27,7 +27,8 @@ type SketchBuilder struct{}
 func (s *SketchBuilder) Run(ctx *types.Context) error {
 	sketchBuildPath := ctx.SketchBuildPath
 	buildProperties := ctx.BuildProperties
-	includes := utils.Map(ctx.IncludeFolders.AsStrings(), utils.WrapWithHyphenI)
+    libFolders := append(ctx.IncludeFolders.AsStrings(), ctx.BuildPath.Join("sketch").Join("libdefs").String()) // user definitions for library include files
+    includes := utils.Map(libFolders, utils.WrapWithHyphenI)
 
 	if err := sketchBuildPath.MkdirAll(); err != nil {
 		return errors.WithStack(err)
diff --git a/test/testdata/sketch_with_multiple_custom_libraries/libdefs/lib2_user_config.h b/test/testdata/sketch_with_multiple_custom_libraries/libdefs/lib2_user_config.h
new file mode 100644
index 00000000000..e8ea284f7ec
--- /dev/null
+++ b/test/testdata/sketch_with_multiple_custom_libraries/libdefs/lib2_user_config.h
@@ -0,0 +1,8 @@
+
+// This user file is in 'sketchDir/libdefs/' so it can be included by libraries.
+//
+// Its name and what to include is specified by the library documentation.
+
+#pragma once
+
+#define LIB2_SOME_CONFIG 1
diff --git a/test/testdata/sketch_with_multiple_custom_libraries/libraries2/Lib/lib2.h b/test/testdata/sketch_with_multiple_custom_libraries/libraries2/Lib/lib2.h
index e69de29bb2d..5cea15b7756 100644
--- a/test/testdata/sketch_with_multiple_custom_libraries/libraries2/Lib/lib2.h
+++ b/test/testdata/sketch_with_multiple_custom_libraries/libraries2/Lib/lib2.h
@@ -0,0 +1,14 @@
+
+#if defined __has_include
+#  if __has_include (<lib2_user_config.h>)
+#    pragma message "Including local project config file <lib2_user_config.h>"
+#    include <lib2_user_config.h>
+#    define DEFAULT_VALUES_ARE_GIVEN 1
+#  endif
+#endif
+
+#ifndef LIB2_SOME_CONFIG
+#define LIB2_SOME_CONFIG 0
+#endif
+
+#define LIB2_SOME_SIZE ((LIB2_SOME_CONFIG) * 42)
diff --git a/test/testdata/sketch_with_multiple_custom_libraries/sketch_with_multiple_custom_libraries.ino b/test/testdata/sketch_with_multiple_custom_libraries/sketch_with_multiple_custom_libraries.ino
index fb99d2a57ee..8633af92e29 100644
--- a/test/testdata/sketch_with_multiple_custom_libraries/sketch_with_multiple_custom_libraries.ino
+++ b/test/testdata/sketch_with_multiple_custom_libraries/sketch_with_multiple_custom_libraries.ino
@@ -1,6 +1,10 @@
 #include "lib1.h"
 #include "lib2.h"
 
+#if LIB2_SOME_SIZE == 42
+#pragma message "this was expected"
+#endif
+
 void setup() {
 }