The 25-year-old was bestowed the honour ahead of the Vitality Blast fixture against Durham, becoming the third Notts player to be capped this season after Ben Slater and Tom Moores.
“The county cap is recognition for significant contributions to the club, and it represents Joe’s transition into a senior player,” said the club’s Director of Cricket Mick Newell.
“Joe has put in a number of performances of real quality across all formats since joining the club, a high proportion of which have contributed to winning matches for the team.
“He’ll continue to grow as a player in the coming years and will do so both with a high ceiling of potential and with remarkable numbers already on the board for a cricketer of his age.”
Clarke joined the Green and Golds at the conclusion of the 2018 season, and has since averaged 32.62 in First-Class cricket for the club.
His 136 from 65 deliveries against Northamptonshire Steelbacks set a Nottinghamshire T20 record.
“Being capped is a marker of player progression and, as a club that has committed to developing young cricketers, that’s something that’s hugely important to us,” added Nottinghamshire Head Coach Peter Moores.
“It stands you out as having gone from a player with potential, to one who is an established part of the Notts set-up.
“It also holds you up alongside a rich history of players who have represented Nottinghamshire in the past, and is a significant accolade."