The 46-year-old Democrat was the US Senate nominee but narrowly lost to Republican rival Senator Ted Cruz in 2018.
He wrote: "I'm really proud of what El Paso did and what El Paso represents".
O'Rourke planned to follow his announcement with a trip to Iowa, the state that will hold the first Democratic nominating contest in February 2020.
Thursday morning's announcement will leave former Vice President Joe Biden as the last big name yet to make a decision about the 2020 campaign. In an encouraging sign, however, a third of respondents said they did not know enough about him to have an opinion.More news: No. 6 Michigan State expects Ward to play in Big Ten Tournament
The former USA congressman enters as a formidable candidate. Kamala Harris of California, Elizabeth Warren of Massachusetts, Bernie Sanders of Vermont and Corey Booker of New Jersey, among others.
He added: "You can probably tell that I want to run. I do".
In 2012, he won against incumbent Democratic congressman Silvestre Reyes in a Latino-majority district, and was re-elected twice before giving up his seat to run against Cruz.
Mr O'Rourke, a former punk musician who sometimes went skateboarding to blow off steam on the Texas campaign trail, has been tipped to quickly achieve rockstar status with a run.