Striker Niasse joins on loan from Everton until the end of the season, while attacking midfielder Evandro has signed on a permanent deal from Porto, with his contract running until 2019.
The additions come as a huge boost to Hull after attacking midfielder Markus Henriksen was ruled out for up to three months with a shoulder injury he sustained against Manchester United on Wednesday (AEDT).
Niasse, 26, will hope to revive his career after an unsuccessful year at Everton.
He signed in a $22.1 million deal from Lokomotiv Moscow last February but only played five times – with two starts – in the Premier League last season.
Things have got worse in 2016-2017 - Niasse has not made a single first-team appearance and played exclusively for the Under-23 side.
Widespread reports suggest Hull has an option to buy him on a permanent $16.3 million deal.
Evandro, meanwhile, arrives after only starting eight league matches in two-and-a-half years at Porto.
The 30-year-old had joined Porto from Estoril, where he impressed under Marco Silva, Hull's new head coach.
Hull is bottom of the Premier League, but remain within three points of safety ahead of their home match with Bournemouth on Sunday.
The club is facing a battle to keep top scorer Robert Snodgrass, who has been linked with West Ham and Middlesbrough as well as a big-money move to China.