The 24-year-old has arrived in Malta on Sunday and has already agreed to personal terms with the Premier League club and has put pen to paper until the end of the season.
The signing of a defender was a priority for new Sta Lucia FC coach Giovanni Tedesco who last week replaced Oliver Spiteri at the helm of the team.
Fonseca is now expected to attend his first training session with his new teammates on Monday evening.
Fonseca is a Cape Verde international defender who has spent the majority of his career in the Netherlands.
He came through the youth ranks of Sparta Rotterdam where he played for the club at U-17 and U-21 level before finally making it into the first team in the summer of 2020.
Fonseca then moved to Den Bosch in February last year before this summer he headed to Portugal where he was on the books of Coimbra.
At Coimbra, Fonseca struggled for regular football and has now decided to restart his career in Malta with Sta Lucia with the aim of helping the club avoid relegation.
Fonseca is the second Dutch-born player to join Sta Lucia this month after they sign striker Meghon Valpoort.