Jennifer Aniston shows off her athletic figure in a new U.S. ad campaign.

The 41-year-old star is plugging electrolyte-enhanced Smart Water and looks immaculate in the black and white photograph.

In the ad, currently running in American magazines, the actress is pictured in her gym gear, baring her taut tummy while clutching a bottle of the mineral water.

On form: Jennifer Aniston in the new Smart Water ad campaign, dressed in her gym gearing and holding a bottle of the electrolyte-enhanced drink

She is pictured above a caption which reads: 'My workout partner. Working out feels great...afterwards.

'That's why I drink Smart. It's hydration I can feel. (And it's one part of my routine I never sweat.)'

Aniston is no stranger to exercise. The yoga devotee also runs to keep in shape.

Smart cookie: Jennifer Aniston gets into bed to advertise the water

My workout partner: Aniston has advertised the brand for many years

Legs to die for: Aniston in Los Angeles last month

But she recently denied she had taken part in an extreme baby food diet to shed the pounds ahead of filming in Hawaii for her new film Just Go With It.

It had earlier been reported that she was the latest star to dry the Baby Food Cleanse diet, created by trainer Tracy Anderson.

The regime involves eating 14 servings of puréed food each day followed by a healthy dinner.

But Aniston told People.com: 'I've been asked lately, "Jen, what's this baby food diet all about?" I kept thinking: "That's the strangest question ever."

'Sorry, but the last time I had baby food, I believe I was . I've been on solids for about 40 years now.'

While evidently looking young for 41, Aniston insisted in a recent interview that she hadn't tried Botox yet, but hadn't ruled it out.

She said: 'I could do it, and I mean these lines are getting deeper every day, but when I tell you what’s happened to me – these lines are just about living.

'Look I eat really well and I work out, but I also indulge when I want to. I don’t starve myself in an extremist way.

'You’re not taking away my coffee or my dairy or my glass of wine because I’d be devastated. My advice: just stop eating s**t every day.'