The collision took place on the cycle track by Victoria Avenue’s Lay-by Three last Thursday (17 February) around 19:00.
Officers said it was high tide and waves were crashing over the sea wall when ambulances arrived.
“A female in her 50s was taken to A&E with serious head injuries and has subsequently been transported to Southampton Hospital for treatment,” they said, adding that she remained “in a stable condition with ongoing support.”
“The other cyclist was treated at the scene by Ambulance staff for minor injuries and did not require hospital treatment.”
Anyone who saw what happened or helped the cyclists at the time is being asked to get in touch with Police on 612612, quoting M01123/2022.