Rasmussen, who signs a four-year deal with the club, leaves Die Königsblauen having never made a first team appearance for them, but he has been a more than prominent player in their U19 side over the last two seasons.
After winning the U19 Bundesliga in 2014-15, the Danish defender made 26 appearances in the competition in 2015-16 - failing to complete 90 minutes just once in the campaign.
Now, he has the opportunity to play senior football with St. Pauli in a bid to further his rapid development.
Delight all round
Speaking to the club's official website, Sporting Director Thomas Meggle and manager Ewald Lienen both expressed their delight with Rasmussen's arrival - Freibeuter der Liga's sixth signing of the summer so far.
The former described Rasmussen as "a promising talent," saying that his high "potential" is something the teenager has "demonstrated" both with Schalke and with Denmark's youth teams, for whom he is a regular.
Lienen, meanwhile, is happy with the "flexibility in the centre of defence" that the 19-year-old will offer St. Pauli.
After conceding 39 goals in 34 2. Bundesliga games last season, it's no surprise the club are looking to bolster their back line, and the manager is confident Rasmussen will do this, adding: "He is strong in the air and in the tackle and has good technique."
As for the player himself, he revealed the reasoning behind his decision to move to St. Pauli, saying that he sees "good opportunities for [him] to develop as a player at the Millerntor."
"I am looking forward to my time at St. Pauli," he continued, adding: "I'll go full out from day one to put myself in contention for as much playing time as possible," with his first real chance to experience senior football there on a plate.