Andersen spent four seasons with the Tykes before completing a move to Kenilworth Road, playing in both the Championship and most recently League One.
He leaves Barnsley having made 167 appearances, establishing himself as one of the best defensive players in the English Football League - featuring in the League One play-off final last season as well as in the Championship play-offs in 2021.
Despite suffering relegating in 2022, the Dane has been earmarked for a move further up the football pyramid for some time and now joins the Hatters alongside former Rotherham winger Chiedozie Ogbene.
“It feels very, very good,” said the centre-back. “I’m veryhappy and I’m really looking forward to this next chapter of my life at Luton Town.
“I’m so excited. It’s a dream come true. When I made my move in Denmark from Brondby to a lower team in the same division, me and my dad spoke about it and I said ‘In one and a half years, I’m going to go to the Championship’, and that is exactly what happened.
“During that period I also wrote on a piece of paper, ‘Premier League’, and stuck it on the fridge. I completely forgot about it, until my family told me, ‘You put that on the fridge!’. So it’s a proud moment for them as well.”
Luton's first season back in the Premier League begins away at Brighton on 12th August 2023.