According to reports, Chelsea will pay around £10 million plus Alonso for the former Arsenal striker.
The deal was completed after the 11 p.m. deadline after Chelsea filed the deal sheet to give more time. Aubameyang arrived in the UK on Thursday afternoon for his medical to complete the deal.
Aubameyang sustained a minor jaw fracture in a weekend attack on his home. The Gabon forward is expected to be out for three weeks, but will resume training next week.
Pierre-Emerick Aubameyang left Arsenal for Barcelona on a free transfer in January.
Thomas Tuchel must be eager to reunite with Aubameyang, a player who scored 79 goals for him in two seasons at Borussia Dortmund
Alonso left Chelsea by mutual consent, giving Barcelona more time to finalize a deal even though they have not yet made an official announcement of his arrival. In his six years with the team, he won six trophies, including the Premier League and Champions League.
The deal means Chelsea have spent a whopping £278.4m during this transfer window, acquiring the services of Wesley Fofana, Marc Cucurella, Kalidou Koulibaly and Raheem Sterling. It is the most any club has spent in a single transfer window.