Niang spent last season on-loan with Alloa Athletic where he made over 30 appearances.
The 22-year-old joins from Partick Thistle and plays as a defensive minded midfielder.
Niang was infamously sent off after just 25 seconds against Hartley’s Cove Rangers side last season and adds further depth to the Pools midfield.
Upon joining the club Niang told the club website: “I am delighted to sign for this great club. I have heard a lot about the Hartlepool fans and how passionate they are.
"I am here to work hard and be a part of the great culture Paul is building here.”
Pools boss Hartley added: “We’re delighted to bring Sena into the club. He’s someone we’ve seen a lot of up in Scotland.
"He’s a hardworking midfielder who can give us options in a few different positions. We’re looking forward to working with Sena ahead of the pre-season camp in Portugal.”
Niang will wear the number four shirt and meet up with the first team squad ahead of the training camp in Portugal.
Meanwhile, Pools could soon be set to add their fourth signing of the summer with Rangers winger Jake Hastie.
Hastie spent the second half of the season on-loan with Northern Irish side Linfield having also had a spell with Partick Thistle.
Hastie can operate on either wing and is said to be close to a deal with Pools with several reports in Scotland, including Football Scotland, suggesting the Rangers winger is set to end his time at Ibrox.
Hastie has not featured for Rangers since 2019 but has had several loan spells away from the club in that time including a period with Rotherham United in League One in 2019 where he scored in each of his first three games for the Millers.
Pools boss Hartley recently told The Mail he was confident there would be a number of new arrivals before the squad heads out to Portugal next week for their warm weather training camp.