This follows the resignation of former ward councillor Tim Wills, after anti-racist campaign group HOPE not Hate said he allegedly sent messages to a Telegram chat associated with Patriotic Alternative.
His resignation left the Conservative group on the council without a majority, although Kevin Jenkins still replaced Dan Humphreys as council leader last month.
Meanwhile Martin McCabe defected from the Lib Dems to Labour last week.
Labour’s Vicki Wells has been elected after the Marine by-election, receiving 1,239 votes.
Conservative Syed Ahmed finished second with 972 votes, the Green Party’s Sonya Mallin polled 145 votes and Lib Dem Emma Norton received 112 votes.
After the result, Ms Wells tweeted: “WE WON!!! Thanks everyone - you’ve all been amazing - we did it!
“Your vote means that Worthing Borough Council is no longer dominated by the complacent Conservatives - who just lost overall control for the first time EVER!”
Beccy Cooper, leader of the Labour group, added: “Huge thanks to Team Labour from far and wide who have pounded pavements with us for Worthing By-election victory.
“Thanks to all. We will work hard to make sure our town thrives and works for all.”
Current composition of the council:
Conservatives: 17, Labour: 17, Liberal Democrats: 2, Independent: 1.
As both parties have the same number of seats either the Conservatives will continue as a minority administration or power could transfer over to Labour, who would also have the task of running a minority administration.
They would both have the chance to secure a majority in May, when 13 of 37 seats will be up for election.
Of those 13 seats, eight are held by Conservatives, four by Labour and one by the Lib Dems.
Today’s by-election result caps of an incredible resurgence by Labour in Worthing over the past three-and-a-half years.
Back in June 2018, Beccy Cooper won a by-election, also in Marine, to become her party’s first representative at WBC for 40 years.