Liberali came through AC Milan's academy before being allowed to leave for Catanzaro permanently last summer, with Milan retaining a 50% sell-on clause as part of that deal.
He went on to make his name in Calabria over the course of last season, performances that turned him into one of the most closely watched young players in the country.
Como moved decisively to beat off competition from elsewhere in Serie A, meeting Catanzaro's release clause in full to secure the deal, with roughly half of the fee due to be passed on to Milan under the terms of their sell-on arrangement.
Head coach Cesc Fabregas welcomed the signing, praising the midfielder's technical quality and composure in possession.
"Mattia is a creative player with excellent technical ability and composure on the ball. He works well in tight spaces, creates chances and contributes in the final third. We believe his qualities are perfect for our system, and we will work to continue his development," Fabregas said in the club's official announcement.
Liberali also spoke to Como's in-house media after putting pen to paper, describing his excitement at the move and outlining what he believes he can bring to the team.
"It's a great feeling to be able to start my journey at Como. I have a real desire to grow and improve, and in that I've found a lot in common with the club. I've also spoken with the coach and I think the playing philosophy is perfect for me. I can't wait to play at the Sinigaglia and get to know the fans," he said.
He also gave supporters a sense of his style on the pitch.
"My main characteristic is my unpredictability, playing good football as a team, helping my teammates, looking for them. I hope to give the fans plenty of enjoyment and I can't wait to see them at the Sinigaglia," Liberali added.
The move continues Como's pattern of investing in high-potential young talent under Fabregas, with the club betting that Liberali's creativity and comfort in tight areas can translate quickly to the level required in Serie A.
The fee reflects how highly Liberali is regarded after his impact at Catanzaro, with several of the league's leading clubs having tracked his progress before Como moved to secure his signature.