The Big Lime Weekend comprises a variety of triathlon events in and around Hartlepool marina from Friday, August 12, to Sunday, August 14.
People of all ages are welcome, but under-16s must be accompanied by an adult.
Marshals will be placed at various points of the triathlon course to assist competitors and spectators.
A number of people have already offered to help, but the council’s Active & Creative Hartlepool service is hoping more volunteers will come forward.
Briefing support and guidance will be provided to all volunteers prior to the events.
Katie Myers, Hartlepool Borough Council’s participation officer, said: “The marshals have a vital role to play both in terms of competitors and spectators.
“This is also an opportunity for people to gain experience of volunteering and being involved in the organisation and running of a large-scale open air event.”
The Big Lime Weekend comprises a 750-metre predictor swim on the Friday in the marina, where competitors try to match their predicted time.
Saturday sees the aquathlon at 9am of a 750-metre open water swim in the marina, followed by a 5km run along the promenade course.
Sunday’s triathlon involves a 750-metre open water swim, a 20km cycle and 5km run.
Online entries close at 5pm on Friday, August 5.
Anyone who is interested in becoming a marshal should email [email protected] and full details will be provided.