Brazilian Evandro, 30, has signed on a two-and-a-half-year deal for an undisclosed fee and teams back up with Hull's new head coach Marco Silva after the pair worked together at Estoril.
Senegal international Niasse, 26, has joined on loan until the end of the season.
Niasse is available to face Bournemouth in Saturday's home Premier League game while Evandro is awaiting international clearance.
The deals bolster a Hull squad that has been further depleted by the loss of Markus Henriksen to injury.
"Markus Henriksen is likely to be sidelined for two to three months with a shoulder injury picked up during Tuesday night's EFL Cup tie with Manchester United," the club stated on their official website.
"The Norwegian international met with a consultant yesterday following scans and will now be out for a prolonged amount of time.
"We wish Markus well with his recovery."
Source : PA