Lascelles signed for the Magpies from Nottingham Forest two years ago, but spent the 2014-15 campaign on loan at the club before officially joining up with his new team.
Following Benitez's appointment as manager in March, Lascelles cemented a regular spot in the team, and now his boss has promoted him to skipper.
"It's an unbelievable feeling for a manager like Rafa Benitez to give me the captain's armband at a club like this," Lascelles told the club's official website. "It is a lot of responsibility for a young player but I will always give my all because I want what is best for the team and the club.
"It is such a proud moment for me and the biggest in my career so far. It is nice for someone to show that much trust in you, and I want to repay the manager's faith by leading this team back into the Premier League."
Benitez added: "We have a few players now in the squad who have the character to be captain, but Jamaal has proven himself to be a leader and a big personality in the dressing room and on the pitch.
"At the end of last season he really developed as a character and a man. I am pleased that Jamaal will be our captain, but I have also said to other players that they must too show their character, strength and leadership qualities to support the team."
Lascelles will lead the team out for their first Championship match of the season away to Fulham tomorrow night.