WASHINGTON — Sen. Jon Kyl, the Arizona Republican appointed to fill the Senate seat left vacant by John McCain's death this year, is resigning from Congress at the end of the month, Arizona Gov. Doug Ducey said on Friday.

“Senator Kyl didn’t need to return to the Senate," Ducey said in a statement. "His legacy as one of Arizona’s most influential and important political figures was already without question. But he did return, and I remain deeply grateful for his willingness to step up and serve again when Arizona needed him."

Kyl’s resignation letter was delivered to Ducey’s office on Thursday afternoon.

"It has been an honor and a privilege to again serve the people of Arizona," Kyl wrote. "When I accepted your appointment, I agreed to complete the work of the 115th Congress and then re-evaluate continuing to serve. I have concluded that it would be best if I resign so that your new appointee can begin the new term with all other Senators in January 2019 and can serve a full two (potentially four) years."

Ducey's office said that a replacement “will be announced in the near future.”

According to Arizona law, the state's governor has the authority to appoint a replacement to an open Senate seat, and the replacement must belong to the same political party as the person being replaced.

Kyl's resignation comes after Democrat Kyrsten Sinema was elected to the Senate seat held by Jeff Flake, R-Ariz., who did not run for re-election. Kyl's seat could be one of Democrats' top pick-up opportunities in the 2020 election.

McCain died in August, and Kyl returned to the Senate at the beginning of September. He had previously served in the Senate as minority whip from 2007 until 2013, and in the House from 1987 until 1995.