In a letter to the secretary of the Senate, Cruz said he was reactivating his Senate campaign now that his presidential bid has ended.
"I will continue fighting for jobs, freedom, and security in the Senate for 27 million Texans and all Americans across this great country. The conservative movement remains strong and vibrant. When conservatives are united, we are far more effective," he said.
Cruz ended his White House bid last week after Donald Trump defeated him by a wide margin in the Indiana primary.
Cruz was elected to the Senate in 2012 as a tea-party insurgent. On Capitol Hill, he has frequently clashed with GOP leaders.
The senator is seen by many backers as a potential candidate for president again, perhaps as soon as 2020.