The Welsh Xavi, as he has popularly become known by fans, signed a five-year deal at the Britannia after the two clubs agreed a figure believed to be around £13m.
The 26-year-old followed Brendan Rodgers from Swansea City to Anfield in 2012, but he did not quite live up to expectations in four years on Merseyside.
Allen made 47 appearances for club and country last season, playing a key role in Wales’ Euro 2016 campaign in France.
But he does not fit in Jurgen Klopp’s plans, departing Liverpool after 132 appearances – 40 of them coming from the bench – and Reds fans wished him well after growing to appreciate his tireless efforts on the field.
See some of the best reaction to his departure, below: