Wilmer Valderrama has reportedly bought an engagement ring for Demi Lovato.

The 'That '70s Show' actor, who secretly rekindled his romance with the former 'X Factor' USA judge earlier this year, has sparked speculation he may propose to the 21-year-old beauty after being spotted buying a diamond sparkler from Tiffany & Co. last month

An eyewitness told RadarOnline.com: ''It looked like an engagement ring, but could have just been a nice expensive ring he was giving to Demi.

''He was really low-key at the store and was trying to stay under the radar.''

The 33-year-old actor allegedly engraved the ring with a special message reading, 'With you and by you always', which he also wrote in a Twitter message to mark Demi's 21st birthday in August.

He wrote: ''I've been so proud of the strength & commitment you've gained 4 life, keep inspiring us all! Happy birthday my @ddlovato! With & by u always (sic)''

The couple have dated on and off since 2010 but have remained quiet about their latest reunion since getting back together in February.

Demi and Wilmer have been spotted out and about numerous times together as late as just last week when the actor hosted a private dinner to celebrate the end of Demi's two-year stint on singing competition 'The X Factor'.

The couple dined with pals at West Hollywood's RivaBella restaurant after Demi finished filming the show's third season finale.