Two vehicles collided on the A32 Gosport Road shortly after 08:00 GMT, Hampshire Constabulary said.
The biker was treated by paramedics at the scene but later died from his injuries.
Motorists are urged to avoid the area as the road, which has been closed near Wych Lane, will remain shut for several hours.

Media playback is not supported on this device
City, unbeaten in the WSL in 2016, beat Chelsea 2-0 on Sunday to win the title with one game to play.
Houghton, 28, joined City with England team-mates Jill Scott, Karen Bardsley and Toni Duggan before the 2014 season.
"Coming here, it was a massive risk for me. In the first year, it was a bit of an unknown," she told BBC Sport.
"We took that risk and we've seen this club grow."
Defender Houghton joined City from Arsenal as they were controversially introduced to the WSL for the 2014 season.
They were placed in the top division as the competition was expanded to two tiers following a bidding process, in a move which saw Doncaster Rovers Belles demoted to WSL 2.
City won the Continental Cup in their first year, were second in the league in 2015 and, on Sunday, secured their first national title in front of a club-record crowd of 4,096 at the Academy Stadium.
"That's what I joined the club for - I knew it was going to be about winning trophies and being ambitious," Houghton added on BBC Radio 5 live sports extra.
"We deserve this. We've been brilliant all season and every player and staff member has been fantastic."
City have kept 16 clean sheets in their 21 matches in all competitions so far in 2016 and will attempt to win another trophy when they face Birmingham City in the Continental Cup final on 2 October.
Their starting line-up against Chelsea was made up entirely of British players - nine English and two Scottish.
City's victory took them 10 points clear of second-placed Chelsea, who won the league and FA Cup double in 2015.
"I'm really pleased that we've won the title - it's what we set out to do," said City manager Nick Cushing.
"But I'm more pleased with the performance today. In the second half especially, we were dominant defensively and offensively."
BBC television commentator Jonathan Pearce on BBC Radio 5 live sports extra:
"Because of the investment they've put into the team - they've really backed women's football at Manchester City - it was inevitable they were going to win the title, whether it was this season or next.
"Right from the word go, they've been very, very strong. They've got a real core contingent of players with England as well and that helps, as they stay together with club and country.
"It comes down to money, investment and belief. I think Chelsea will come back fighting and they'll have players with points to prove who will come back very, very strongly.
"But, unless the other clubs do something remarkable, it will be the big three - and I'm not including Liverpool in that. Unless Liverpool really work hard in terms of investment, then their days of winning WSL have gone, I think, for a while."
A motorcyclist has died after being involved in a crash with a lorry in Fareham, Hampshire.
[NEXT_CONCEPT]
England captain Steph Houghton believes her "risky" move to Manchester City has been justified by their first Women's Super League title.