John Stamos has worn a bald cap to show "solidarity" with Dave Coulier

Sharing photos of himself shaving his friend's head on Instagram, he wrote: "Nothing like throwing on a bald cap and flexing some Photoshop skills to show some love and solidarity with my bro @dcoulier.

"You're handling this with so much strength and positivity - it's inspiring. I know you're going to get through this, and I'm proud to stand with you every step of the way. I love you."

And he added a sweet tribute to Dave's wife Melissa Coulier, describing her as "the most wonderfulest" and his pal's "true life line".

She replied to the post: "We love you, so much."

John and Dave have been close friends since starring on the 1990s sitcom together.

When Dave shared his diagnosis, John immediately promised to be by his side.

He wrote on Instagram last week: "My brother from day 1. Love you @dcoulier and I'll be by your side through it all."

The 65-year-old actor was diagnosed just five weeks before he revealed the news publicly.

He told 'TODAY' host Hoda Kotb: "In that time, I've had three surgeries, I've had chemo, I've lost a little bit of hair.

"I kind of look like a little baby bird now, but it has been a roller coaster ride for sure."

Dave revealed he is battling "a very aggressive type of lymphoma", but he's glad that the cancer hasn't spread.

He added: "At that point, the curability rate went up to 90 plus percent. So it's very treatable."

And he had told his wife he would be "OK with whatever the news" is amid his health battle.

He said: "I can't explain where that came from. I've had an incredible life. I've had the most amazing people in my life.

"This has been an extraordinary journey, and I'm OK if this is the end of the journey."