To mark the special occasion, the doting parents were quick to share a touching montage of photos and video clips over on their Instagram grids.
Amongst the slew of snaps, former pro footballer David, 48, and fashion designer VB, 49, shared a string of heartmelting throwback photos from Romeo's childhood.
The sweet compilation featured clips of Romeo on holiday, snippets of the youngster playing football and more recent pictures of their son cosying up to his model girlfriend, Mia Regan.
Alongside the birthday tribute, former Spice Girl Victoria proudly gushed: "Happy Birthday @RomeoBeckham…. The sweetest, kindest and most generous soul. We are so proud of you and we love you more than words can express, you are our everything xx Kissess."
Doting dad David, meanwhile, penned: "Happy 21st Birthday to my little man [pink heart emoji] Dad is so proud of the person that you have become, kind, generous and passionate about what you love , keep being you and dream big [pink heart emoji] we love u so much x @romeobeckham x."
Fans and friends flooded the comments section with an array of heartfelt messages. One commented: "You know you're a good father when your kiddo wants to be just like you," while another wrote: "@davidbeckham from one father to another. You seem like a super dad!"
Commenting on VB's post, one follower penned: "What a lovely loving family you all are. I'm not normally one to comment but your pics make me smile," and a second sweetly added: "They grow up so quickly."
Extending the birthday celebrations, David and Victoria treated Romeo to a special celebratory dinner. Whilst eldest son Brooklyn didn't make an appearance, budding footballer Romeo was joined by his younger sister Harper, 12, his younger brother Cruz, 18, in addition to his girlfriend, Mia, 20.
The family ended the evening with a special rendition of happy birthday. And in a clip shared to Victoria's Instagram Stories, a joyful Romeo could be seen beaming from ear to ear as a waiter presented him with a chocolate pudding adorned with a tall pink candle.
Musing on his birthday dinner, Romeo gushed: "Most beautiful night with my beautiful family [pair of black heart emojis] love you all so much @mimimoocher @davidbeckham @cruzbeckham."
It's been a busy time for the Beckham clan who recently returned from their family holiday to Croatia.
From sunset guitar sessions to lavish yacht trips, the famous family appeared to enjoy an idyllic break.
Sharing a glimpse inside their holiday, Victoria posted a steamy snapshot of herself passionately kissing beau David alongside the caption: "Family time is everything [loved-up emoji] Happy weekend, kisses from us all xxxx."