"I'm back, because I give a damn," the South Australian lawyer said on his campaign website, launched on Thursday.
Mr Xenophon was first elected to the Senate for SA in 2007 and was re-elected in 2013 and 2016.
He resigned in October 2017 after he announced he was unsure whether he was a dual citizen, which would have made him ineligible to run for parliament.
However, the High Court later found Mr Xenophon was validly elected and able to run for office.
He turned his attention to South Australian politics, running for a lower house in the 2018 state election under the SA Best party, but failed.
Mr Xenophon said in a statement he had "tried to be a political hermit" for four years.
"I can no longer sit on the sidelines," he said.
The odds are stacked against the former senator, as he won't have a party to put his name above the line on the upper house ballot paper - a position only given to groups and parties.
"I won't have the resources of the major parties. I won't have their powerful donors," he said.
"But what I do have is a burning passion and energy to fight for our state; for the issues that matter and to give everyday South Australians a genuine voice."
He will be up against two of his former advisers, senators Stirling Griff and Rex Patrick.
However, Senator Patrick said he would make an announcement on Friday regarding his political intentions.
"Clearly Nick Xenophon's decision to throw his hat into the SA Senate changes the landscape," he said in a statement.
"I have worked for Nick as an adviser and I have great respect for him as a politician. He can again be a fine representative of our great state."
Mr Xenophon was renowned for his colourful stunts to gain attention for a range of issues from defence spending to the social impacts of gambling.
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