diff --git a/pkg-r/NAMESPACE b/pkg-r/NAMESPACE
index 56116f09..5bb459a5 100644
--- a/pkg-r/NAMESPACE
+++ b/pkg-r/NAMESPACE
@@ -10,6 +10,7 @@ export(chat_mod_ui)
 export(chat_ui)
 export(markdown_stream)
 export(output_markdown_stream)
+export(update_chat_user_input)
 if (getRversion() < "4.3.0") importFrom("S7", "@")
 import(S7)
 import(rlang)
diff --git a/pkg-r/NEWS.md b/pkg-r/NEWS.md
index 941e15b1..7b9d9d60 100644
--- a/pkg-r/NEWS.md
+++ b/pkg-r/NEWS.md
@@ -1,8 +1,11 @@
 # shinychat (development version)
 
-## New features and improvements
+## New features
 
 * Added `chat_enable_bookmarking()` which adds Shiny bookmarking hooks to save and restore the `{ellmer}` chat client. (#28)
+* Added `update_chat_user_input()` for programmatically updating the user input of a chat UI element. (#78)
+
+## Improvements
 
 * `chat_app()` now correctly restores the chat client state when refreshing the app, e.g. by reloading the page. (#71)
 
diff --git a/pkg-r/R/chat.R b/pkg-r/R/chat.R
index 8683a701..8237e8a9 100644
--- a/pkg-r/R/chat.R
+++ b/pkg-r/R/chat.R
@@ -541,3 +541,83 @@ chat_clear <- function(id, session = getDefaultReactiveDomain()) {
     )
   )
 }
+
+
+#' Update the user input of a chat control
+#'
+#' @param id The ID of the chat element
+#' @param ... Currently unused, but reserved for future use.
+#' @param value The value to set the user input to. If `NULL`, the input will not be updated.
+#' @param placeholder The placeholder text for the user input
+#' @param submit Whether to automatically submit the text for the user. Requires `value`.
+#' @param focus Whether to move focus to the input element. Requires `value`.
+#' @param session The Shiny session object
+#'
+#' @export
+#' @examplesIf interactive()
+#' library(shiny)
+#' library(bslib)
+#' library(shinychat)
+#'
+#' ui <- page_fillable(
+#'   chat_ui("chat"),
+#'   layout_columns(
+#'     fill = FALSE,
+#'     actionButton("update_placeholder", "Update placeholder"),
+#'     actionButton("update_value", "Update user input")
+#'   )
+#' )
+#'
+#' server <- function(input, output, session) {
+#'   observeEvent(input$update_placeholder, {
+#'     update_chat_user_input("chat", placeholder = "New placeholder text")
+#'   })
+#'
+#'   observeEvent(input$update_value, {
+#'     update_chat_user_input("chat", value = "New user input", focus = TRUE)
+#'   })
+#'
+#'   observeEvent(input$chat_user_input, {
+#'     response <- paste0("You said: ", input$chat_user_input)
+#'     chat_append("chat", response)
+#'   })
+#' }
+#'
+#' shinyApp(ui, server)
+
+update_chat_user_input <- function(
+  id,
+  ...,
+  value = NULL,
+  placeholder = NULL,
+  submit = FALSE,
+  focus = FALSE,
+  session = getDefaultReactiveDomain()
+) {
+  rlang::check_dots_empty()
+  check_active_session(session)
+
+  if (is.null(value) && (submit || focus)) {
+    rlang::abort(
+      "An input `value` must be provided when `submit` or `focus` are `TRUE`."
+    )
+  }
+
+  vals <- drop_nulls(
+    list(
+      value = value,
+      placeholder = placeholder,
+      submit = submit,
+      focus = focus
+    )
+  )
+
+  session$sendCustomMessage(
+    "shinyChatMessage",
+    list(
+      id = resolve_id(id, session),
+      handler = "shiny-chat-update-user-input",
+      obj = vals
+    )
+  )
+}
diff --git a/pkg-r/R/utils.R b/pkg-r/R/utils.R
index e1373871..ca4a09d9 100644
--- a/pkg-r/R/utils.R
+++ b/pkg-r/R/utils.R
@@ -18,3 +18,8 @@ strip_ansi <- function(text) {
   ansi_pattern <- "(\x1B|\x033)\\[[0-9;?=<>]*[@-~]"
   gsub(ansi_pattern, "", text)
 }
+
+
+drop_nulls <- function(x) {
+  x[!vapply(x, is.null, FUN.VALUE = logical(1))]
+}
diff --git a/pkg-r/man/update_chat_user_input.Rd b/pkg-r/man/update_chat_user_input.Rd
new file mode 100644
index 00000000..828a546f
--- /dev/null
+++ b/pkg-r/man/update_chat_user_input.Rd
@@ -0,0 +1,67 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/chat.R
+\name{update_chat_user_input}
+\alias{update_chat_user_input}
+\title{Update the user input of a chat control}
+\usage{
+update_chat_user_input(
+  id,
+  ...,
+  value = NULL,
+  placeholder = NULL,
+  submit = FALSE,
+  focus = FALSE,
+  session = getDefaultReactiveDomain()
+)
+}
+\arguments{
+\item{id}{The ID of the chat element}
+
+\item{...}{Currently unused, but reserved for future use.}
+
+\item{value}{The value to set the user input to. If \code{NULL}, the input will not be updated.}
+
+\item{placeholder}{The placeholder text for the user input}
+
+\item{submit}{Whether to automatically submit the text for the user. Requires \code{value}.}
+
+\item{focus}{Whether to move focus to the input element. Requires \code{value}.}
+
+\item{session}{The Shiny session object}
+}
+\description{
+Update the user input of a chat control
+}
+\examples{
+\dontshow{if (interactive()) (if (getRversion() >= "3.4") withAutoprint else force)(\{ # examplesIf}
+library(shiny)
+library(bslib)
+library(shinychat)
+
+ui <- page_fillable(
+  chat_ui("chat"),
+  layout_columns(
+    fill = FALSE,
+    actionButton("update_placeholder", "Update placeholder"),
+    actionButton("update_value", "Update user input")
+  )
+)
+
+server <- function(input, output, session) {
+  observeEvent(input$update_placeholder, {
+    update_chat_user_input("chat", placeholder = "New placeholder text")
+  })
+
+  observeEvent(input$update_value, {
+    update_chat_user_input("chat", value = "New user input", focus = TRUE)
+  })
+
+  observeEvent(input$chat_user_input, {
+    response <- paste0("You said: ", input$chat_user_input)
+    chat_append("chat", response)
+  })
+}
+
+shinyApp(ui, server)
+\dontshow{\}) # examplesIf}
+}