Katie Archibald, 28, said the veteran cross country racer had gone into cardiac arrest while they were lying in bed together.
The champion cyclist said she “can’t describe” the pain of his death.
In an emotional statement on Twitter, she said: “I still don’t understand what’s happened; if this is real; why he’d be taken now – so healthy and happy.”
“He went into cardiac arrest while we were lying in bed. I tried and tried, and the paramedics arrived within minutes, but his heart stopped and they couldn’t bring him back.”
“Mine stopped with it. I love him so much and need him here with me. I need him here so badly, but he’s gone. I can’t describe this pain.”
Katie, who won track cycling golds with teammate Dame Laura Kenny at the Rio 2016 and Tokyo 2020 Olympics, added: “Thank you to those making tributes. I can’t bear to talk about him in the past tense to say anything myself. You mean everything to me, Rab. I love you.”
Wardell had won the elite title at the Scottish MTB XC Championships at Kirroughtree Forest in Dumfries and Galloway at the weekend. The former Commonwealth Games cyclist appeared on BBC Scotland on Monday, just hours before his death.
Scottish Cycling said it was “devastated devas- to confirm the news that international mountain biker and former employee Rab Wardell has passed away”.
The sports body added: “We send our love and support to his family, friends and all those in our community who knew him.”
Olympic gold medallist Sir Chris Hoy was among those sending messages of support. He said: “Can’t believe this. Utterly heartbreaking.”
Former Olympic cyclist and Tour de France yellow jersey wearer David Millar said the Scotsman’s death was “properly hard to comprehend”.
British Cycling said: “Rab was a brilliant rider, friend and ambassador for our sport, and will be sorely missed by so many.”