diff --git a/Makefile.am b/Makefile.am
index 8e5f72b1..6c7d2d3f 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -66,6 +66,11 @@ if IS_HAIKU
 AM_CPPFLAGS += -D_BSD_SOURCE
 endif
 
+# Set flags for Cygwin.
+if IS_CYGWIN
+AM_CPPFLAGS += -DTMUX_SOCK_PERM=0
+endif
+
 # List of sources.
 dist_tmux_SOURCES = \
 	alerts.c \
diff --git a/configure.ac b/configure.ac
index b63d7e11..4f90fcd5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -982,6 +982,7 @@ AM_CONDITIONAL(IS_NETBSD, test "x$PLATFORM" = xnetbsd)
 AM_CONDITIONAL(IS_OPENBSD, test "x$PLATFORM" = xopenbsd)
 AM_CONDITIONAL(IS_SUNOS, test "x$PLATFORM" = xsunos)
 AM_CONDITIONAL(IS_HPUX, test "x$PLATFORM" = xhpux)
+AM_CONDITIONAL(IS_CYGWIN, test "x$PLATFORM" = xcygwin)
 AM_CONDITIONAL(IS_HAIKU, test "x$PLATFORM" = xhaiku)
 AM_CONDITIONAL(IS_UNKNOWN, test "x$PLATFORM" = xunknown)