The blonde beauty, who has been in headlines this week due to her romance with actress Michelle Rodriguez, called her long time friend "strong" and "truly inspiring" in the image caption.
CLICK ON PHOTOS FOR GALLERYShe posted a picture of the pair posing together, and wrote: "Happy birthday @badgalriri This picture means so much to me because it was the first time we met and you have been such an incredible friend ever since."
Cara continued: "You are such a strong and truly inspiring woman and i am so lucky to have you in my life! I love you boo and thank you for supporting me through so much ❤"
Rihanna is currently in Aspen celebrating her 26th birthday, and the social media-loving celebrity shared some pictures from her special day with her fans.
The Rude boy singer can be seen enjoying cake and cookies and champagne, as well as posing in the snow in a bikini and biker jacket, in one snap titled "cabin life".
Riri was without her BFF as she celebrated her birthday since Cara was in the midst of Milan Fashion Week.
The Burberry model was joined in Italy by her new girlfriend though. Michelle confirmed the two are dating, telling the Mirror: "It’s going really well. She’s so cool."
She added: "When we started hanging out I just thought she was awesome, and we have the best time together."
Cara spoke about Michelle at the Elle awards, telling Digital Spy: "I love her, she's great."