News the deal had been done came just after the Premier League's summer transfer deadline expired on Thursday:
Lo Celso will play for Spurs on loan during the 2019-20 campaign, and the Premier League side has an option to make the move permanent.
The player is Tottenham's second midfield acquisition of the summer, having landed Tanguy Ndombele from Lyon earlier in the window.
Spurs have been linked with a number of attacking midfielders in recent weeks, including Sporting CP's Bruno Fernandes. However, they have moved to secure Lo Celso in a transfer that should excite supporters.
Spanish football journalist Andy West said despite links to Philippe Coutinho and Paulo Dybala, Lo Celso's arrival shouldn't be viewed as a consolation by Spurs fans:
The player joined Betis on loan from Paris Saint-Germain last season, and the Andalusian club took up the option to make the transfer permanent this summer. That was after he enjoyed a breakthrough season in Spain.
At PSG, Lo Celso was often used in a deeper position, but he was given more freedom at Betis. He excelled in 2018-19 as a result, joining attacks, getting into the box and making tangible contributions for his team.
Eleven Sports looked at some of his best moments from his solitary year in Spain:
Squawka Football noted how important he was to the Betis cause:
It will be interesting to see how Lo Celso is deployed by Mauricio Pochettino at Spurs, as they are already blessed with a number of attacking-midfield options.
Dele Alli and Christian Eriksen both like to play in support of the centre-forward, whether that be Harry Kane or Heung-Min Son. Further back in midfield, Spurs now have Ndombele to call on, as well as Harry Winks, Moussa Sissoko and Victor Wanyama.
Lo Celso, like a number of players have done already, will seek to push his game on to another level under Pochettino. With his career on an upward trajectory, the 23-year-old has the potential to be one of the most exciting moves made by any team in the Premier League.