The Tottenham Hotspur youngster began life as a defender, but has impressed since being moved further upfield by Mauricio Pochettino at the start of the campaign - going on to make the position his own for both club and country.
Dier, who was raised in Portugal and came through the ranks at Sporting CP where he also featured in midfield, made his England debut there in November and starred in March's friendly win against Germany.
The 22-year-old was named as one of 11 midfielders in Roy Hodgson's squad for Euro 2016, though the decision to call up only three central defenders has prompted suggestions that Dier revert to his previous role in the event of emergency.
And Barnes, who played 79 times for England, believes the back four is home for Dier.
"He’s a centre-back anyway," the former Liverpool winger told the Alan Brazil Sports Breakfast show on Tuesday (c. 09:10).
"As much as he did well against Germany in central midfield and that’s where he’s been playing for Tottenham, I don’t necessarily see it as his long-term position."
Dier played 50 times for Spurs in 2015-16, though only four of those appearances came as a centre-back.