diff --git a/NEWS.md b/NEWS.md
index 6aae9977..442e2f66 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,5 +1,7 @@
 # stringr (development version)
 
+* Add `sep` argument to `str_dup()` so that it is possible to repeat a string and
+  add a separator between every repeated value (@edward-burn, #564).
 * `str_*` now errors if `pattern` includes any `NA`s (@nash-delcamp-slp, #546).
 * `str_view()` now displays a message when called with a zero-length character
   vector (@LouisMPenrod, #497).
diff --git a/R/dup.R b/R/dup.R
index d90e62fb..3a82ffe0 100644
--- a/R/dup.R
+++ b/R/dup.R
@@ -5,14 +5,24 @@
 #'
 #' @inheritParams str_detect
 #' @param times Number of times to duplicate each string.
+#' @param sep String to insert between each duplicate.
 #' @return A character vector the same length as `string`/`times`.
 #' @export
 #' @examples
 #' fruit <- c("apple", "pear", "banana")
 #' str_dup(fruit, 2)
+#' str_dup(fruit, 2, sep = " ")
 #' str_dup(fruit, 1:3)
 #' str_c("ba", str_dup("na", 0:5))
-str_dup <- function(string, times) {
-  vctrs::vec_size_common(string = string, times = times)
-  stri_dup(string, times)
+str_dup <- function(string, times, sep = NULL) {
+  input <- vctrs::vec_recycle_common(string = string, times = times)
+  check_string(sep, allow_null = TRUE)
+
+  if (is.null(sep)) {
+    stri_dup(input$string, input$times)
+  } else {
+    map_chr(seq_along(input$string), function(i) {
+      paste(rep(string[[i]], input$times[[i]]), collapse = sep)
+    })
+  }
 }
diff --git a/man/str_dup.Rd b/man/str_dup.Rd
index fb722f2c..48a7867b 100644
--- a/man/str_dup.Rd
+++ b/man/str_dup.Rd
@@ -4,13 +4,15 @@
 \alias{str_dup}
 \title{Duplicate a string}
 \usage{
-str_dup(string, times)
+str_dup(string, times, sep = NULL)
 }
 \arguments{
 \item{string}{Input vector. Either a character vector, or something
 coercible to one.}
 
 \item{times}{Number of times to duplicate each string.}
+
+\item{sep}{String to insert between each duplicate.}
 }
 \value{
 A character vector the same length as \code{string}/\code{times}.
@@ -22,6 +24,7 @@ A character vector the same length as \code{string}/\code{times}.
 \examples{
 fruit <- c("apple", "pear", "banana")
 str_dup(fruit, 2)
+str_dup(fruit, 2, sep = " ")
 str_dup(fruit, 1:3)
 str_c("ba", str_dup("na", 0:5))
 }
diff --git a/tests/testthat/_snaps/dup.md b/tests/testthat/_snaps/dup.md
new file mode 100644
index 00000000..08a752c2
--- /dev/null
+++ b/tests/testthat/_snaps/dup.md
@@ -0,0 +1,13 @@
+# separator must be a single string
+
+    Code
+      str_dup("a", 3, sep = 1)
+    Condition
+      Error in `str_dup()`:
+      ! `sep` must be a single string or `NULL`, not the number 1.
+    Code
+      str_dup("a", 3, sep = c("-", ";"))
+    Condition
+      Error in `str_dup()`:
+      ! `sep` must be a single string or `NULL`, not a character vector.
+
diff --git a/tests/testthat/test-dup.R b/tests/testthat/test-dup.R
index 2872ae37..ca7ddbb9 100644
--- a/tests/testthat/test-dup.R
+++ b/tests/testthat/test-dup.R
@@ -13,3 +13,21 @@ test_that("0 duplicates equals empty string", {
 test_that("uses tidyverse recycling rules", {
   expect_error(str_dup(1:2, 1:3), class = "vctrs_error_incompatible_size")
 })
+
+test_that("uses sep argument", {
+  expect_equal(str_dup("abc", 1, sep = "-"), "abc")
+  expect_equal(str_dup("abc", 2, sep = "-"), "abc-abc")
+
+  expect_equal(str_dup(c("a", "b"), 2, sep = "-"), c("a-a", "b-b"))
+  expect_equal(str_dup(c("a", "b"), c(1, 2), sep = "-"), c("a", "b-b"))
+
+  expect_equal(str_dup(character(), 1, sep = "-"), character())
+  expect_equal(str_dup(character(), 2, sep = "-"), character())
+})
+
+test_that("separator must be a single string", {
+  expect_snapshot(error = TRUE, {
+    str_dup("a", 3, sep = 1)
+    str_dup("a", 3, sep = c("-", ";"))
+  })
+})