diff --git a/.ci/linters/dt_lengths_linter.R b/.ci/linters/dt_lengths_linter.R
new file mode 100644
index 000000000..3ca218c4e
--- /dev/null
+++ b/.ci/linters/dt_lengths_linter.R
@@ -0,0 +1,13 @@
+# This is lintr::lengths_linter(), but including 'vapply_1i()' which is more common internally in data.table
+# alternatively, this could be defined with something like
+#   dt_lengths_linter <- lintr::lengths_linter()
+#   local({ # to remove 'e'
+#     e <- environment(dt_lengths_linter)
+#     e$function_names <- c(evalq(function_names, e), "vapply_1i")
+#   })
+#   but that feels more fragile than this.
+dt_lengths_linter <- lintr::make_linter_from_function_xpath(
+  function_names = c("sapply", "vapply", "vapply_1i", "map_int", "map_dbl"),
+  xpath = "parent::expr/parent::expr[expr/SYMBOL[text() = 'length']]",
+  lint_message = "Use lengths() to find the length of each element in a list."
+)