The fearless reporter is among a number of high profile media figures who are leaving the independent broadcaster.
Ms Halligan was recently awarded a national order of merit by France, where she was praised for being a woman of conviction.
The native of Templeogue, Dublin, entered journalism in the 1990s, firstly with the Sunday Tribune and then with Magill magazine.
More recently, the television journalist was praised for her courage in coming out as gay during the same-sex marriage referendum campaign, where she talked about hiding this part of her identity for years.
The head of 3News Mick McCaffrey announced Ms Halligan’s departure yesterday and highlighted her honest and formidable reporting for the station.
“Ursula has made a valuable contribution to TV3 over the past 18 years and was always an independent voice in politics, going beyond the spin and hype to give our viewers the real story of what was happening at Leinster House.”
Mr McCaffrey said the station’s political editor commanded admiration among peers and politicians.
“She was respected across all political parties for her insights and independence. We would like to thank Ursula for her dedicated service and wish her the very best for the future.”
Ms Halligan also presented her own show, The Political Party, for five years.
She will leave TV3 at the end of August, having worked at the station for almost 19 years.
The announcement of her departure comes just after Vincent Browne announced he is stepping down as presenter of the Tonight programme from the end of July.
Ms Halligan famously once asked former taoiseach Brian Cowen if he was aware that the internet was alive with the belief that he had been drunk or hungover in an RTÉ radio interview.
She was also involved in an incident which became known as ‘flowerpot gate’ where advisers to Enda Kenny collided with her as reporters attempted to quiz the then-taoiseach about same-sex marriage.
Ms Halligan disclosed in a broadcast in 2009 how then-finance minster Brian Lenihan had been diagnosed with pancreatic cancer.