Casspi and the Kings reached an agreement on a $6 million, two-year deal last week. Terms of Mbah a Moute's contract were not immediately known.
Casspi had several big games with the Kings last season after George Karl took over as coach in February. Mbah a Moute averaged a career-high 9.9 points to go with 4.9 rebounds per game with Philadelphia last season.
The Kings signed point guard Rajon Rondo, shooting guard Marco Belinelli and center Kosta Koufas on Monday. Veteran forward Caron Butler has yet to sign his agreement, which is expected to be completed this week.
Mbah a Moute and Casspi are both in their second stints in Sacramento.
The Kings acquired Mbah a Moute from Milwaukee in the summer of 2013, but they traded him to Minnesota for Derrick Williams less than a month into the season. Williams signed with the New York Knicks last week.
The 28-year-old Mbah a Moute was born in Cameroon and played in college at UCLA. The Bucks drafted him 37th overall in 2008. He has averaged 6.8 and 4.9 rebounds in seven NBA seasons.
Mbah a Moute posted a picture on Instagram signing his contract with Sacramento and wrote: "Back to the Kingdom!!"
The Kings drafted the Israel-born Casspi with the 23rd pick in 2009. He spent his first two years with Sacramento before playing two seasons with Cleveland and one with Houston.
The Kings signed Casspi again last summer after he was waived by New Orleans. Casspi came on strong under Karl late last season, averaging 19.9 points in the final eight games.
The 27-year-old Casspi wanted to return to Sacramento and waited out the first week of free agency. He posted a picture on Instagram of him signing his contract with the headline: "A King for two more!"