The 21-year-old joined Bristol City’s academy in 2019 and progressed to the first team, making his senior debut against Cardiff City in 2023.
He later spent successful loan spells with Crewe Alexandra and Swindon Town, making 38 and 35 appearances respectively.
Knight-Lebel enjoyed an impressive campaign at Crewe, scoring three goals, before earning his first Canada call-up in October 2024.
He made his international debut against Suriname in November and has since collected three caps.
"I want to welcome Jamie to the club," Motherwell boss Alfred Johansson said.
"He is a talented young player who has two very strong seasons out on loan playing regularly. A physical and strong defender, he will provide competition in the defensive positions and bring international experience.
"Bristol City rate him highly, and this loan move will provide him with the opportunity to play in a style suited to his game. We look forward to introducing Jamie to the squad, and I'm excited to be working with him."