Capable of playing at left-back or further forward, the 21-year-old will provide competition and quality down the flank.
Last season, Lewis spent time at French side Amiens, playing nine times in Ligue 2, and English League One side Plymouth where he featured on 20 occasions.
He has one first-team appearance for Liverpool.
Livi manager David Martindale revealed he will likely impact games further up the park such is the threat he possesses with his left foot.
“I am excited to bring a player of Adam’s quality in to the club,” he told Livingston's website.
"From the first phone call that led to me researching the player, I have had nothing but positivity about the lad. I really like what I have seen from the footage and the opinions I have had back on the player have all been very good.
“Adam can play anywhere on the left-hand side but has played a lot at left-back. I think with the qualities he possesses, we will see him higher up the park influencing the game in the opponent’s half.
“He has a great left foot and can look after the ball in tight situations. He is a competitor with a will to win and fits right into the ethos at the club.
“He is taking a big chance by moving his young family up to Scotland which speaks volumes for the lad. I have no doubt we will be a good fit for one another.
“It’s fantastic to be trusted with players of this calibre from one of the biggest football clubs in the world and hopefully this leads to further Liverpool FC prospects making their way up to Livingston FC. It’s a win-win for both parties as we get a player whom financially would not be on our radar and the player gets some much needed game time to progress and enhance their development in a very competitive league.”
“Going forward, I haven’t seen anyone as good as him at that age, in terms of quality and what he can deliver in the final third,” he said.