diff --git a/NAMESPACE b/NAMESPACE
index 9c782ac5..d716832d 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -58,6 +58,7 @@ export(route)
 export(routeQueryCheck)
 export(scrub_key)
 export(set_ggmap_option)
+export(set_map_alpha)
 export(showing_key)
 export(theme_inset)
 export(theme_nothing)
diff --git a/R/set_map_alpha.R b/R/set_map_alpha.R
new file mode 100644
index 00000000..eb26ec7e
--- /dev/null
+++ b/R/set_map_alpha.R
@@ -0,0 +1,54 @@
+#' Add transparency to a ggmap object
+#'
+#' Add an alpha (transparency) layer to a \code{ggmap} object. This makes it
+#' possible to superimpose multiple map layers and have the lower layers shine
+#' through.
+#'
+#' @param map A \code{ggmap} object.
+#' @param alpha The required transparency (a number between 0 and 1).
+#'
+#' @return A \code{ggmap} object.
+#' @export
+#'
+#' @examples
+#' zoom <- 12
+#' bbox <- c(-0.3275, 51.407222, 0.0725, 51.607222)
+#'
+#' base <- get_map(bbox, zoom, maptype = "terrain-lines")
+#' overlay <- get_map(bbox, zoom, maptype = "watercolor")
+#'
+#' # Plot the base map.
+#' ggmap(base)
+#' # Plot the overlay map (100% opacity).
+#' ggmap(overlay)
+#'
+#' # Plot the base map with the overlay superimposed at 25% opacity.
+#' ggmap(base) +
+#'   inset_ggmap(
+#'     set_map_alpha(overlay, 0.25)
+#'   )
+set_map_alpha <- function(map, alpha) {
+  if(class(map)[1] != "ggmap") {
+    stop("map must be a ggmap object", call. = FALSE)
+  }
+  if(class(alpha) != "numeric" || alpha < 0 || alpha > 1) {
+    stop("alpha must be a number between 0 and 1", call. = FALSE)
+  }
+
+  # Record the attributes & dimensions of the map object.
+  map_attributes <- attributes(map)
+  map_dimensions <- dim(map)
+
+  # Add an alpha channel.
+  map <- grDevices::adjustcolor(map, alpha)
+
+  # Add back the dimensions (convert vector to matrix).
+  dim(map) <- map_dimensions
+  # Add back the attributes.
+  attributes(map) <- map_attributes
+
+  # Convert from matrix to map.
+  class(map) <- c("ggmap", "raster")
+
+  map
+}
diff --git a/man/set_map_alpha.Rd b/man/set_map_alpha.Rd
new file mode 100644
index 00000000..1fa60c1f
--- /dev/null
+++ b/man/set_map_alpha.Rd
@@ -0,0 +1,39 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/set_map_alpha.R
+\name{set_map_alpha}
+\alias{set_map_alpha}
+\title{Add transparency to a ggmap object}
+\usage{
+set_map_alpha(map, alpha)
+}
+\arguments{
+\item{map}{A \code{ggmap} object.}
+
+\item{alpha}{The required transparency (a number between 0 and 1).}
+}
+\value{
+A \code{ggmap} object.
+}
+\description{
+Add an alpha (transparency) layer to a \code{ggmap} object. This makes it
+possible to superimpose multiple map layers and have the lower layers shine
+through.
+}
+\examples{
+zoom <- 12
+bbox <- c(-0.3275, 51.407222, 0.0725, 51.607222)
+
+base <- get_map(bbox, zoom, maptype = "terrain-lines")
+overlay <- get_map(bbox, zoom, maptype = "watercolor")
+
+# Plot the base map.
+ggmap(base)
+# Plot the overlay map (100\% opacity).
+ggmap(overlay)
+
+# Plot the base map with the overlay superimposed at 25\% opacity.
+ggmap(base) +
+  inset_ggmap(
+    set_map_alpha(overlay, 0.25)
+  )
+}
diff --git a/tests/testthat/test-set-map-alpha.R b/tests/testthat/test-set-map-alpha.R
new file mode 100644
index 00000000..ea5fab68
--- /dev/null
+++ b/tests/testthat/test-set-map-alpha.R
@@ -0,0 +1,8 @@
+context("alpha")
+source("util.R")
+
+test_that("adds alpha layer", {
+  map <- getFakeMap()
+  map <- set_map_alpha(map, 0.5)
+  expect_identical(as.character(map), rep("#FFFFFF80", 4))
+})
diff --git a/tests/testthat/util.R b/tests/testthat/util.R
index 5067ee39..3d29a572 100644
--- a/tests/testthat/util.R
+++ b/tests/testthat/util.R
@@ -1,5 +1,7 @@
 getFakeMap <- function() {
-  map <- character()
+  map <- rep("#FFFFFF", 4)
+  dim(map) <- c(2, 2)
+
   class(map) <- c('ggmap','raster')
   attr(map, "source")  <- "osm"
   attr(map, "maptype") <- "openstreetmap"
@@ -9,4 +11,4 @@ getFakeMap <- function() {
     ur.lat = 3, ur.lon = 4
   )
   map
-}
\ No newline at end of file
+}