Robin Thicke's girlfriend April Love Geary has poked fun at the couple's age gap on Instagram.

The 21-year-old beauty - who started dating the 'Blurred Lines' hitmaker after he split from his ex-wife, actress Paula Patton - has taken to her Instagram account to share a picture of the couple on Valentine's Day (14.02.17) in which she sarcastically referred to herself as Robin's ''much younger girlfriend''.

Alongside a photograph of the 39-year-old star and herself in Napa Valley, California, April wrote: ''Robin Thicke & his ''much younger girlfriend'' enjoying Napa Valley for Valentine's Day. (sic)''

Robin also marked the annual occasion by sharing a picture of them together, which he captioned: ''Happy Valentines Day from Napa Valley! Hold on tight to the ones you love. I love u all (sic)''

Meanwhile, Robin is currently locked in a bitter custody battle with his ex-wife Paula over their son Julian Fuego.

In January, a judge gave Paula sole legal and physical custody of their six-year-old son after she alleged in court documents that her ex-husband physically abused her and was unfaithful.

Under the temporary restraining order taken out against him, Robin is required to stay 100 yards away from Paula, her mother and Julian outside of the pre-arranged visitation hours.

However, an insider recently said Robin remains ''focused'' on his son despite the Hollywood star's best efforts to damage his public image.

The source said: ''[Paula] keeps trying to bury him, and this is a concerted effort to prejudice the public against him. He has taken no such campaign against her.

''He's taking this extremely seriously, and it's unfortunate and very difficult when you're on the receiving end of a campaign of negativity.''

The 41-year-old beauty divorced Robin in 2015 after 21 years together and almost nine years of marriage.