All old fixtures and fittings – originally installed when the centre was opened in 1992 – have been replaced with up-to-date versions.
And improved facilities include a private showering are and redesigned toilets, as well as new lockers and cubicles.
The changing facilities have also been made more accessible for people with disabilities, including automatic doors, dedicated changing cubicles, lowered vanity surfaces and a rebuilt accessible toilet, shower and changing area.
The work was funded by Hambleton District Council, which provided £80,000, and Sport England, with a further £160,000.
Council leader Mark Robson said: “Thirsk’s swimmers have been waiting a number of years for improvement following investment in the gym and dryside in 2010.
“Having waited to secure Sport England funding, we’ve managed to deliver a top quality refurbishment which will benefit the centre and users for years to come.”
The work is the first in a series of investments in changing facilities at the council’s leisure centres. Stokesley Leisure Centre’s dryside changing areas are being revamped completely and work at Bedale Leisure Centre will start next month to completely replace the wetside shower and toilet facilities.
More information about works at all leisure centres can be found at hambletonleisure.co.uk.