Despite the early inclement weather, the gruelling all-day event has proved hugely popular with spectators lining the banks of Waterloo Lake and the cycle and running routes.
The race began at 6am with swimmers taking to the lake for two loops totalling 2.4 miles.
From there they climbed out of the water onto their saddles for a leg-burning 112-mile three-loop court on their bikes, before running a strength-sapping four-loop spectator-friendly course over 26 miles - marathon length.
Still ongoing, it is expected to continue for up to 17 hours.
Ironman and triathlon events have seen a resurgence in recent years, particularly in West Yorkshire thanks to the home-grown success of Leeds-born brothers Alistair and Jonny Brownlee, who dominated the sportin the 2010s and picked up numerous Olympic medals and world championship titles.
And due to its predicted popularity, roads were closed all around Roundhay Park today to accommodate the route and the sheer volume of spectators.
7. Roundhay Ironman Competition
Thousands gather on the banks to watch the competition. | Steve Riding
11. Roundhay Ironman Competition
Spectators for the swimming which started at 6am. | Steve Riding
