diff --git a/NEWS.md b/NEWS.md
index ba64b6d462..61683895da 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -19,6 +19,8 @@
 
 * Closed arrows in `element_line()` are now filled (@yutannihilation, #2924).
 
+* ggplot2 now works in Turkish locale (@yutannihilation, #3011).
+
 # ggplot2 3.1.0
 
 ## Breaking changes
diff --git a/R/fortify-multcomp.r b/R/fortify-multcomp.r
index ff054c0e72..8aa165eeb3 100644
--- a/R/fortify-multcomp.r
+++ b/R/fortify-multcomp.r
@@ -46,7 +46,7 @@ fortify.glht <- function(model, data, ...) {
 #' @export
 fortify.confint.glht <- function(model, data, ...) {
   coef <- model$confint
-  colnames(coef) <- tolower(colnames(coef))
+  colnames(coef) <- to_lower_ascii(colnames(coef))
 
   plyr::unrowname(base::data.frame(
     lhs = rownames(coef),
diff --git a/R/layer.r b/R/layer.r
index 7994670713..e172f7fa3b 100644
--- a/R/layer.r
+++ b/R/layer.r
@@ -341,7 +341,7 @@ is.layer <- function(x) inherits(x, "Layer")
 
 
 check_subclass <- function(x, subclass,
-                           argname = tolower(subclass),
+                           argname = to_lower_ascii(subclass),
                            env = parent.frame()) {
   if (inherits(x, subclass)) {
     x
diff --git a/R/save.r b/R/save.r
index fdddc7eeac..7f2609d312 100644
--- a/R/save.r
+++ b/R/save.r
@@ -145,7 +145,7 @@ plot_dev <- function(device, filename = NULL, dpi = 300) {
   )
 
   if (is.null(device)) {
-    device <- tolower(tools::file_ext(filename))
+    device <- to_lower_ascii(tools::file_ext(filename))
   }
 
   if (!is.character(device) || length(device) != 1) {
diff --git a/R/stat-ydensity.r b/R/stat-ydensity.r
index 6f3769c61e..978f60ad0c 100644
--- a/R/stat-ydensity.r
+++ b/R/stat-ydensity.r
@@ -110,7 +110,7 @@ calc_bw <- function(x, bw) {
     if (length(x) < 2)
       stop("need at least 2 points to select a bandwidth automatically", call. = FALSE)
     bw <- switch(
-      tolower(bw),
+      to_lower_ascii(bw),
       nrd0 = stats::bw.nrd0(x),
       nrd = stats::bw.nrd(x),
       ucv = stats::bw.ucv(x),
diff --git a/R/utilities.r b/R/utilities.r
index 1e2d687c93..9b422e0d74 100644
--- a/R/utilities.r
+++ b/R/utilities.r
@@ -302,6 +302,20 @@ has_name <- function(x) {
   !is.na(nms) & nms != ""
 }
 
+# Use chartr() for safety since toupper() fails to convert i to I in Turkish locale
+lower_ascii <- "abcdefghijklmnopqrstuvwxyz"
+upper_ascii <- "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+to_lower_ascii <- function(x) chartr(upper_ascii, lower_ascii, x)
+to_upper_ascii <- function(x) chartr(lower_ascii, upper_ascii, x)
+
+tolower <- function(x) {
+  stop('Please use `to_lower_ascii()`, which works fine in all locales.', call. = FALSE)
+}
+
+toupper <- function(x) {
+  stop('Please use `to_upper_ascii()`, which works fine in all locales.', call. = FALSE)
+}
+
 # Convert a snake_case string to camelCase
 camelize <- function(x, first = FALSE) {
   x <- gsub("_(.)", "\\U\\1", x, perl = TRUE)
@@ -313,11 +327,11 @@ snakeize <- function(x) {
   x <- gsub("([A-Za-z])([A-Z])([a-z])", "\\1_\\2\\3", x)
   x <- gsub(".", "_", x, fixed = TRUE)
   x <- gsub("([a-z])([A-Z])", "\\1_\\2", x)
-  tolower(x)
+  to_lower_ascii(x)
 }
 
 firstUpper <- function(s) {
-  paste(toupper(substring(s, 1,1)), substring(s, 2), sep = "")
+  paste0(to_upper_ascii(substring(s, 1, 1)), substring(s, 2))
 }
 
 snake_class <- function(x) {