The London-born centre-back has been a stalwart of the Lilywhites' youth setup since joining the club, making 99 appearances for the club's Under-21s since making his debut at that level in the 2017-18 season.
Primarily a central defender, Lyons-Foster was also deployed as a full-back and defensive midfielder for the Lilywhites' academy teams and gleaned European experience in the UEFA Youth League.
Over the past couple of years, Lyons-Foster has frequently sported the captain's armband for Tottenham's Under-21s, scoring three goals and setting up three more in his 99 fixtures, including the winner against Stevenage in a thrilling 4-3 EFL Trophy success in 2021.
That same year, the defender was included in two Europa Conference League squads by Antonio Conte, who named him on the bench for their opening qualifying defeat to Pacos de Ferreira.
Lyons-Foster made the matchday squad again for a group-stage loss to Vitesse in October 2021, but he failed to earn any minutes in either game and has not received another senior call-up since.
The 23-year-old has continued to prove his worth at youth level with one assist from 14 appearances across the Premier League 2 and EFL Trophy, helping Spurs win each of their opening 11 league games of the new season.
However, having failed to make his senior debut for the Lilywhites, Lyons-Foster will now advance his career in Scandinavia, as confirmed in a statement from Tottenham: "We have reached agreement with HJK Helsinki for the transfer of Brooklyn Lyons-Foster.
"A product of our Academy system, versatile defender Brooklyn has risen through the ranks to become a staple of our Development Squad in recent years, captaining our Premier League 2 side on numerous occasions.
"Now 23 years of age, he was named as a substitute in two competitive first team matches during the 2021-22 campaign. We thank Brooklyn for his contributions to our Academy and wish him well for the future."
HJK - who won their fourth successive Finnish top-flight title in 2023 - confirmed that Lyons-Foster has signed a two-year contract with the option of a further 12 months until 2027, and the centre-back professed that there is a "reason for everything" when reflecting on his failure to make the grade in North London.
"Tottenham is of course a dear club to me. I have spent more than half of my life in the club, growing both as a player and as a person," Lyons-Foster told the HJK website.
"It certainly feels strange for a while that I'm not representing Tottenham anymore, but on the other hand, it's wonderful to be able to start my own journey now. I'm really, really excited about this opportunity and can't wait to get started.
"Of course, it was a big disappointment, but I believe that there is a reason for everything. Now I am happy to be able to start in a new club with a long and successful history and European games and a fight for the championship ahead."
Tottenham will not receive a transfer fee for Lyons-Foster's exit, but the Lilywhites have negotiated a sell-on clause, allowing them to pocket a percentage of any future fee.