diff --git a/hugr-core/src/builder/dataflow.rs b/hugr-core/src/builder/dataflow.rs
index bf70268d8..04ec38b4b 100644
--- a/hugr-core/src/builder/dataflow.rs
+++ b/hugr-core/src/builder/dataflow.rs
@@ -1,13 +1,18 @@
+use itertools::Itertools;
+
 use super::build_traits::{HugrBuilder, SubContainer};
 use super::handle::BuildHandle;
 use super::{BuildError, Container, Dataflow, DfgID, FuncID};
 
 use std::marker::PhantomData;
 
+use crate::hugr::internal::HugrMutInternals;
 use crate::hugr::{HugrView, ValidationError};
 use crate::ops;
+use crate::ops::{DataflowParent, Input, Output};
+use crate::{Direction, IncomingPort, OutgoingPort, Wire};
 
-use crate::types::{PolyFuncType, Signature};
+use crate::types::{PolyFuncType, Signature, Type};
 
 use crate::extension::ExtensionRegistry;
 use crate::Node;
@@ -155,6 +160,109 @@ impl FunctionBuilder<Hugr> {
         let db = DFGBuilder::create_with_io(base, root, body)?;
         Ok(Self::from_dfg_builder(db))
     }
+
+    /// Add a new input to the function being constructed.
+    ///
+    /// Returns the new wire from the input node.
+    pub fn add_input(&mut self, input_type: Type) -> Wire {
+        let [inp_node, _] = self.io();
+
+        // Update the parent's root type
+        let new_optype = self.update_fn_signature(|mut s| {
+            s.input.to_mut().push(input_type);
+            s
+        });
+
+        // Update the inner input node
+        let types = new_optype.signature.body().input.clone();
+        self.hugr_mut()
+            .replace_op(inp_node, Input { types })
+            .unwrap();
+        let mut new_port = self.hugr_mut().add_ports(inp_node, Direction::Outgoing, 1);
+        let new_port = new_port.next().unwrap();
+
+        // The last port in an input/output node is an order edge port, so we must shift any connections to it.
+        let new_value_port: OutgoingPort = (new_port - 1).into();
+        let new_order_port: OutgoingPort = new_port.into();
+        let order_edge_targets = self
+            .hugr()
+            .linked_inputs(inp_node, new_value_port)
+            .collect_vec();
+        self.hugr_mut().disconnect(inp_node, new_value_port);
+        for (tgt_node, tgt_port) in order_edge_targets {
+            self.hugr_mut()
+                .connect(inp_node, new_order_port, tgt_node, tgt_port);
+        }
+
+        // Update the builder metadata
+        self.0.num_in_wires += 1;
+
+        self.input_wires().last().unwrap()
+    }
+
+    /// Add a new output to the function being constructed.
+    pub fn add_output(&mut self, output_type: Type) {
+        let [_, out_node] = self.io();
+
+        // Update the parent's root type
+        let new_optype = self.update_fn_signature(|mut s| {
+            s.output.to_mut().push(output_type);
+            s
+        });
+
+        // Update the inner input node
+        let types = new_optype.signature.body().output.clone();
+        self.hugr_mut()
+            .replace_op(out_node, Output { types })
+            .unwrap();
+        let mut new_port = self.hugr_mut().add_ports(out_node, Direction::Incoming, 1);
+        let new_port = new_port.next().unwrap();
+
+        // The last port in an input/output node is an order edge port, so we must shift any connections to it.
+        let new_value_port: IncomingPort = (new_port - 1).into();
+        let new_order_port: IncomingPort = new_port.into();
+        let order_edge_sources = self
+            .hugr()
+            .linked_outputs(out_node, new_value_port)
+            .collect_vec();
+        self.hugr_mut().disconnect(out_node, new_value_port);
+        for (src_node, src_port) in order_edge_sources {
+            self.hugr_mut()
+                .connect(src_node, src_port, out_node, new_order_port);
+        }
+
+        // Update the builder metadata
+        self.0.num_out_wires += 1;
+    }
+
+    /// Update the function builder's parent signature.
+    ///
+    /// Internal function used in [add_input] and [add_output].
+    ///
+    /// Does not update the input and output nodes.
+    ///
+    /// Returns a reference to the new optype.
+    fn update_fn_signature(&mut self, f: impl FnOnce(Signature) -> Signature) -> &ops::FuncDefn {
+        let parent = self.container_node();
+        let old_optype = self
+            .hugr()
+            .get_optype(parent)
+            .as_func_defn()
+            .expect("FunctionBuilder node must be a FuncDefn");
+        let signature = old_optype.inner_signature();
+        let name = old_optype.name.clone();
+        self.hugr_mut()
+            .replace_op(
+                parent,
+                ops::FuncDefn {
+                    signature: f(signature).into(),
+                    name,
+                },
+            )
+            .expect("Could not replace FunctionBuilder operation");
+
+        self.hugr().get_optype(parent).as_func_defn().unwrap()
+    }
 }
 
 impl<B: AsMut<Hugr> + AsRef<Hugr>, T> Container for DFGWrapper<B, T> {
@@ -199,6 +307,7 @@ impl<T> HugrBuilder for DFGWrapper<Hugr, T> {
 #[cfg(test)]
 pub(crate) mod test {
     use cool_asserts::assert_matches;
+    use ops::OpParent;
     use rstest::rstest;
     use serde_json::json;
 
@@ -344,6 +453,38 @@ pub(crate) mod test {
         assert_matches!(builder(), Ok(_));
     }
 
+    #[test]
+    fn add_inputs_outputs() {
+        let builder = || -> Result<(Hugr, Node), BuildError> {
+            let mut f_build = FunctionBuilder::new(
+                "main",
+                Signature::new(type_row![BIT], type_row![BIT]).with_prelude(),
+            )?;
+            let f_node = f_build.container_node();
+
+            let [i0] = f_build.input_wires_arr();
+            let noop0 = f_build.add_dataflow_op(Noop(BIT), [i0])?;
+
+            // Some some order edges
+            f_build.set_order(&f_build.io()[0], &noop0.node());
+            f_build.set_order(&noop0.node(), &f_build.io()[1]);
+
+            // Add a new input and output, and connect them with a noop in between
+            f_build.add_output(QB);
+            let i1 = f_build.add_input(QB);
+            let noop1 = f_build.add_dataflow_op(Noop(QB), [i1])?;
+
+            let hugr =
+                f_build.finish_prelude_hugr_with_outputs([noop0.out_wire(0), noop1.out_wire(0)])?;
+            Ok((hugr, f_node))
+        };
+
+        let (hugr, f_node) = builder().unwrap_or_else(|e| panic!("{e}"));
+
+        let func_sig = hugr.get_optype(f_node).inner_function_type().unwrap();
+        assert_eq!(func_sig.io(), (&type_row![BIT, QB], &type_row![BIT, QB]));
+    }
+
     #[test]
     fn error_on_linear_inter_graph_edge() -> Result<(), BuildError> {
         let mut f_build =
diff --git a/hugr-core/src/hugr/internal.rs b/hugr-core/src/hugr/internal.rs
index b8ac050c3..6062e4084 100644
--- a/hugr-core/src/hugr/internal.rs
+++ b/hugr-core/src/hugr/internal.rs
@@ -74,6 +74,8 @@ pub trait HugrMutInternals: RootTagged {
     /// The `direction` parameter specifies whether to add ports to the incoming
     /// or outgoing list.
     ///
+    /// Returns the range of newly created ports.
+    ///
     /// # Panics
     ///
     /// If the node is not in the graph.
@@ -129,6 +131,9 @@ pub trait HugrMutInternals: RootTagged {
     /// Replace the OpType at node and return the old OpType.
     /// In general this invalidates the ports, which may need to be resized to
     /// match the OpType signature.
+    ///
+    /// Returns the old OpType.
+    ///
     /// TODO: Add a version which ignores input extensions
     ///
     /// # Errors