After defeating England by a field goal in extra time of their semi-final, Samoa became the first nation outside of Australia, New Zealand or Great Britain/England to qualify for the tournament final since 1968.
Johnson, who is half Samoan, shared his words of wisdom with Junior Paulo's side ahead of their historic meeting with Australia at Old Trafford on Saturday (Sunday 3am AEDT).
"I am delivering the message with boundless love and boundless reverence and respect and boundless pride for my boys, my Usos, the Toa Samoa," he said in a video to his 348 million Instagram followers.
"This is a big deal, they are making history in the world of sports, in the world of rugby. This is the first time that our country, our culture has ever gone to the final for any sport.
"I could not be more proud of them, we could not be more proud of them."
The Fast & Furious actor and former wrestler gave himself goosebumps with an address focusing on the importance of grit under pressure.
"I understand what it's like to have pressure," he said.
"There's a few things I always think about.
"Number one is grit ... there's a term Fa'a Samoa.
"My grandfather always taught me that Fa'a Samoa is proud of where you come from, who you are, what's in your blood, what's in your DNA.
"That's the thing that separates us from everyone else but it also means grit."
Johnson said Samoa had the chance to define their legacy this weekend.
"History is watching but our ancestors are watching so when you take that field this Saturday in Manchester and you have that grit and your DNA is full of our Samoan pride and culture and you think about that legacy," he said.
"There's two sides to legacy, number one is the side that you've already made history.
"The other side to legacy, and this is the special side, is when you take that field in Manchester, you leave it all out on the field.
"Grit, legacy, my Usos, I love you, I'm so proud of you."
Samoan players including Joseph Suaalii, Jarome Luai, Anthony Milford and Brian To'o thanked Johnson for his social media message.
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