Joshua Allies, 25, proposed to Jade Clark, 25, on sand dunes at West Beach, near the West Beach Café in Rope Walk, while the aeroplane flew overhead.
He picked Friday (September 4) because they got together seven years ago on this date.
An Air Ads plane flew from their base in Blackpool to circle the Littlehampton skies with the message 'Jade will you marry me' at around 10.30am.
And it all paid off, with Jade saying yes in front of their family and friends.
Jade said she was 'amazed' when she first spotted the plane, and Joshua added: "I never wanted the proposal to be me going down on one knee in a curry house or something. I want it to be something cool, and hopefully a bit romantic."
Speaking ahead of the proposal, Joshua said he was feeling 'totally pumped up and ready' to pop the question to his 'stunning', 'attentive', 'petite and funky' girlfriend.
"When you know, you know, and there is absolutely no doubt in my mind whatsoever that I want to marry her," he said.
The pair met when a friend of Joshua's set them up on a date, meeting at Littlehampton railway station and going for a walk to the beach - a mirror image of the proposal.
The pair traded in jobs at House of Fraser and a local gym to move to the Isle of Wight, where Joshua qualified as a boat captain and Jade trained to be a chef and stewardess.
The couple have spent the last three years touring the Mediterranean aboard the Craftsmans Art, a 24-metre boat which they run together.
Due to coronavirus, the boat's owner asked them to sail back to home waters, and Joshua, from Rustington, seized upon the chance to propose to Jade, from Middleton-on-Sea, at home surrounded by family and friends.
Only a small group knew Joshua's plans, and it had been a challenge to keep it under wraps.
"Jade has Find My Friends on her phone, which lets her see where everybody is, so I've had to be like a secret agent and put my phone on airplane mode at times," he said.
Despite having to come up with increasingly elaborate excuses while planning the event and buying the ring - such as 'helping a friend move a washing machine' - he managed to keep it a surprise.