The £19.8m pre-tax loss for the year to May 2016 was largely due to cost of signing big-name players and a payoff for Brendan Rodgers, who was sacked as manager in October 2015.
Liverpool signed 12 players in the period, spending £32m on striker Christian Benteke, who has since left, and £29m on Brazilian forward Roberto Firmino.
Revenue rose £3m to £301m.
The loss compared with a £60m profit for the previous year, which was largely due to the £75m sale of Luis Suarez to Barcelona.
The club also made £49m from the sale of Raheem Sterling to Manchester City.
Commercial revenue fell £700,000 to £116m, but match day income increased by £3.4m to £62m following a pre-season tour to Australia and Asia. The team also reached the League Cup and Europa League finals.
Media revenue increased by £1m to £124m.
Chief operating officer Andy Hughes said the club, which has been owned by US firm Fenway Sports  Group since October 2010, continued to make "solid financial progress".
"We have seen continued investment in the playing squad, the expanded Main Stand, the new flagship retail store opening later this year, fully refurbished retail stores in Liverpool and Belfast, and we are consulting on a proposed development at our academy in Kirkby to bring together the first team and our young players," he said.
The club said it expected "significantly improved" results for the current financial year.
It has hired Peter Moore, chief competition officer at gaming firm Electronic Arts, to take over as chief executive in June.
The club hopes the 61-year-old - an avid Liverpool fan - can draw on his marketing expertise to boost commercial income.
Liverpool Football Club has fallen into the red despite posting record revenue.