City Councillor Ricardo Arroyo, along with his father, who serves as Suffolk County’s register of probate, on Thursday offered their formal backing to Acting Mayor Kim Janey.

The Hyde Park councillor’s endorsement isn’t much of a surprise: Arroyo has been among Janey’s allies on the Boston City Council during battles over the budget and other issues.

Three members of the City Council are running for mayor alongside Janey, who is seeking a full four-year term after becoming acting mayor in March: Michelle Wu, Annissa Essaibi George and Andrea Campbell. Former Mayor Marty Walsh’s economic development chief John Barros is also a candidate.

The twin endorsements from Councillor Arroyo and Felix D. Arroyo, the register of probate, come two days after state Rep. Jon Santiago, who would have become the first Latino mayor of Boston if he’d won, dropped out.

“I first met Kim over 20 years ago when I was a member of the Boston School Committee, where I saw her advocacy for quality public schools for all students,” Arroyo, the register since 2014, said. “As a member of the Boston City Council and now as Mayor, Kim has continued to fight for justice and equity for all. I know Kim’s heart and I know her values. She is the right person to lead our City, and I will be working hard to elect her to a full term.

Felix D. Arroyo also served on the City Council between 2003 and 2007. He was the first Latino elected to the 13-member body. Another son, Felix G. Arroyo, served on the City Council and gave up his seat for an unsuccessful run for mayor in 2013.

In his own statement, Ricardo Arroyo, who represents District 5, called Janey a “strong progressive partner in our work to address racial and systemic inequities.”

The Arroyos plan to join Janey at an event touting their endorsement, and kicking off a weekend canvassing effort, on Saturday, July 17, in Hyde Park.