"Adam is a really exciting signing for us, someone I am sure the supporters will be really looking forward to seeing play as and when things are able to begin to return to normal," manager Graham Potter said.
"To have both his experience and quality out on the pitch will be a great addition for us, and I know he will be an excellent role model for our younger players in the squad. I am really looking forward to working with him when we return for preseason training in a few weeks."
Lallana, 32, started his career with Southampton and spent a brief loan spell at Bournemouth before he established himself in the Saints' first team. He moved to Liverpool for around £25 million in 2014, where he collected honors in the Champions League, Premier League, FIFA Club World Cup, and UEFA Super Cup.
The playmaker, who will wear No. 14 for Brighton, made 15 appearances during Liverpool's victorious 2019-20 top-flight campaign.