The Irishman’s new contract runs until June 2021, and he will now join up with Neil Harris’s team at The Den, as he makes his first loan move since joining the Seagulls in October 2015.
Under-23 coach Simon Rusk said, “This move comes at the right time for Jayson, and his new deal is well deserved after plenty of hard work over the past few years.
“Jayson has always had the desire to take his game to the next level, and the opportunity to play senior football in the second tier will certainly give him the chance to do that.
“The Championship is one of the most competitive leagues in the world, and we hope Jayson makes the very most of his time at Millwall during the upcoming season.”
Molumby arrived on the south coast from Railway Athletic in 2015, and after impressing for Albion’s under-18s and under-23s, the midfielder made his senior debut in the EFL Cup against Barnet in August 2017.
Another senior appearance followed in the following round of the competition, as Molumby played 120 minutes in Albion’s exit to Premier League opponents AFC Bournemouth at the Vitality Stadium.
The energetic midfielder has also been named on the bench in four Premier League matches for the Seagulls, including last season’s closer against back-to-back champions Manchester City at the Amex Stadium.
Internationally, Molumby captained Republic of Ireland’s under-21s at this summer’s Toulon Tournament, and received a trophy for being one of the competition’s four best players.