"As everyone knows, President Trump and I have had our ups and downs," Efe news quoted Sessions as saying in a statement posted on his campaign website on Saturday.
"But here's the important part: the President is doing great work for America."
Sessions sought to remind voters of his own allegiance to the president, noting his early backing in the 2016 race.
"When President Trump took on Washington, only one Senator out of a hundred had the courage to stand with him: me," his statement read. "I was the first to support President Trump. I was his strongest advocate. I still am. We must make America great again."
Sessions left the Senate to become Attorney General under Trump and pursued a conservative agenda. But he angered the President by recusing himself from the investigation into Russia's interference in the 2016 US presidential election, and Trump repeatedly criticized him for the move.
Sessions, who turns 73 years old next month, resigned exactly a year ago on Thursday. His entry into the race comes a day before Friday's filing deadline.
Alabama's Democratic Senator Doug Jones, widely seen as vulnerable given the state's dominant Republican electorate, shrugged off the news that Sessions planned to run for his seat.
"You need to talk to his opponents in the Republican primary," Jones said. "They're the ones that have their hair on fire right now. I'm not gonna change anything I'm doing."
Republican rivals of Sessions proclaimed loyalty to Trump.
"From the Mueller investigation to this impeachment sham, President Trump has been under constant attack," said Representative Bradley Byrne, one of several candidates.
"I won't sit back and watch them destroy our country. Alabama deserves a senator who will stand with the president and won't run away and hide from the fight."
Byrne has the most cash on hand out of all the Republican candidates, over $2.5 million. Sessions will enter the race with about $2.4 million in his old campaign account, which he can tap for his new Senate bid under federal rules.
Another Republican candidate, former Auburn University football coach Tommy Tuberville, released an ad showing Trump's regrets over choosing Sessions as Attorney General and casting the former Senator as a "DC insider".
Former Alabama Supreme Court Chief Justice Roy Moore, who lost to Jones in the 2017 race, is also competing for the Republican nomination.
( With inputs from IANS )