“I’m so f***ing proud of you guys,” Mr O’Rourke told his supporters during his concession speak on Tuesday night.
He went on to thank “everyone who worked on this campaign, every volunteer, every ambassador, everyone who knocked on doors, everyone who made phone calls, everyone who allowed themselves to hope and to believe and be inspired by one another and turn that into action and into votes”.
The swear word was broadcast live on MSNBC and C-SPAN.
MSNBC’s anchor, Brian Williams, apologised on behalf of the station: “Sorry for the F-bomb. We have no control of what’s in the concession speeches.”
Mr O’Rourke campaigned fiercely in his attempt to push the incumbent from his seat, visiting staunchly conservative parts of the state that his party had long since given up on.
He also shattered fundraising records despite shunning donations from outside political groups and pollster advice.
But in the end, a visit from Donald Trump appeared to help secure Mr Cruz’s victory.
The Republican incumbent argued his opponent’s support for gun control and universal health care were too liberal for Texas.