Liverpool told Barcelona that Colombia international Diaz, one of its key players in winning the Premier League title last season, is not for sale.
The 28-year-old joined Liverpool from Porto in January 2022 and has a contract with the club until 2027.
He scored 13 goals and made seven assists in the league as Liverpool won the title by 10 points.
Diaz attracted interest from Manchester City last summer and also has admirers in Saudi Arabia. He is currently on international duty.
Speculation about Diaz’s future increased after he and his girlfriend both wrote lengthy posts on social media to Liverpool fans that could be interpreted as farewell messages.
Liverpool sporting director Richard Hughes and Fenway Sports Group’s chief executive of football, Michael Edwards, have looked to refresh Arne Slot’s squad early in the transfer window.
Last week, the Premier League champions completed the £29.5 million signing of Dutch right-back Jeremie Frimpong from Bayer Leverkusen.
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Liverpool are close to agreeing to a club-record £109m fee to sign Germany midfielder Florian Wirtz from Leverkusen and are in talks to sign Bournemouth left-back Milos Kerkez for between £45m and £50m.
Georgian goalkeeper Giorgi Mamardashvili will join the squad after joining Liverpool last summer but spending the season on loan at Valencia.
In April, prolific forward Mohamed Salah ended speculation about his future by signing a new two-year contract to keep him at Anfield until 2027, while captain Virgil van Dijk signed a new deal later that month.
Trent Alexander-Arnold has joined Real Madrid one month before the end of his Liverpool contract, while goalkeeper Caoimhín Kelleher has been sold to Brentford for an initial £12.5m fee.
Striker Darwin Núñez has been linked with moves to Saudi Arabian clubs as well as Barcelona.