The Arsenal-owned forward scored two headers against the Seleccao but his efforts ultimately proved in vain, with Silvestre Varela netting a late winner for the Iberians
After seeing his side fall behind 2-0 inside the opening 36 minutes, Bendtner dragged Morten Olsen's men right back into the game with a header shortly before the interval.
The striker netted again with 10 minutes to go to seemingly snatch what would have been a precious point for the Scandinavians, but Silvestre Varela fired home shortly before the full-time whistle to propel the Portuguese into second place in the standings, above Denmark on their head-to-head record.
"I'm really disappointed," Bendtner told DR1. "It's always great to score but the most important thing for me was to hold onto the point.
"My dream of scoring a goal in the Euros came true, but that was the only thing that I was happy about. The disappointment of losing the point was greater than scoring the goals.
"We gave everything we had and we fought really hard to get back into the game. Coming back to 2-2 was fantastic for us. So of course we were sad and disappointed that we lost the game."
Denmark must now attempt to get something out of their final group game, against leaders Germany, and hope that Portugal's clash with Netherlands goes the way of the Dutch.
Bendtner, meanwhile, could be in trouble with Uefa after lifting up his shirt after scoring his equaliser to reveal the name of a well-known Irish bookmaker emblazoned upon his boxer shorts.
Under the governing body's guidelines, players are not permitted to display any advertising on their personal attire during matches.