The signing of Dennis was the biggest in Forest's history, while the signing of Kouyate was a free agent.
The deals were announced over the weekend.
Fourteen new signings have arrived at the City Ground.
Dennis scored 10 league goals in 35 appearances for the Hornets.
He started both of the Hornets' first two games of the season, but did not play in Friday's win.
After his contract at Palace ended in July, he became a free agent and signed a two year deal. He played in 31 games for the Eagles last year.
Forest have been linked with a Atalanta player.
The England keeper on loan from Manchester United is one of the new recruits at Forest.
The club signed Taiwo Awoniyi from Union Berlin for a reported 17 million dollars.
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