The surprise outcome - after Mr Salmond was made favourite to triumph in the prime-time encounter - came after the First Minister struggled to answer repeated questions about the possible currency of an independent Scotland.
Both sides claimed victory as the audience of 350 people, drawn from equal numbers of Yes and No supporters, drifted away from the Royal Conservatoire of Scotland in Glasgow at the end of the most dramatic night yet in the long independence campaign.
However, Mr Darling's team appeared more upbeat given the pressure on Mr Salmond to give the trailing Yes campaign a boost as the battle enters its final six weeks.
A snap ICM/Guardian online poll of 512 Scottish viewers scored the eagerly-awaited contest 56 per cent to 44 per cent in favour of the Better Together leader.
The poll showed 93 per cent of No supporters believed Mr Darling had won, compared with 82 per cent of Yes supporters calling the contest for Mr Salmond.
Mr Darling also won on the arguments - but Mr Salmond was rated as having the stronger personality, by 47 per cent to 39 per cent.
However, though showing a victory for Mr Darling on the night, the poll suggested the debate may not have much impact on people's voting intentions.
After the clash, 53 per cent said they would vote No next month, unchanged from the start.
In a separate straw poll by broadcaster STV, a panel of five undecided audience members also declared Mr Darling the victor.
Better Together campaign director Blair McDougall said the debate "will prove to be a huge, possibly decisive, moment in the referendum campaign".
Blair Jenkins, head of rivals Yes Scotland, said: "This was a clear win for the Yes campaign - a positive, optimistic and visionary case presented by the First Minister against another dose of negativity and scaremongering from Mr Darling."
Hundreds of thousands of people were expected to tune in for the two-hour special programme, although viewing figures were not available last night. Mr Salmond entered the debate buoyed by a new Ipsos-Mori poll for STV showing a surge in support for independence but he was facing a heavy weight of expectation after supporters built it up as a possible game-changer.
The liveliest exchanges came as Mr Darling and Mr Salmond cross-examined each other during the middle section of the show.
On six separate occasions, Mr Salmond sidestepped questions on his "Plan B" for the currency of an independent Scotland, should his proposal to share sterling in a formal pact with the rest of the UK be rejected.
He insisted his currency union plan was "logical and desirable" and dismissed as a "campaign tactic" warnings by the main pro-UK parties they would refuse to sign up if Scotland became independent.
However, in a withering attack, Mr Darling described the currency union plan as "stupidity on stilts".
Mr Salmond used his turn to accuse Better Together of "scaremongering," asking repeatedly why the campaign called itself "Project Fear".
Mr Darling denied the charge - a reference to a comment by a campaign insider last year - and reacted sharply after further allegations were put to him, including a suggestion Scots may have to drive on the right hand side of the road after a Yes vote.
"If we are going to have a bar room chat, a series of jokes, we can do an awful lot better than that," the former Chancellor said.
Earlier, both leaders appeared calm and confident as they delivered their opening statements in the first section of the debate.
The Ipsos Mori poll put support for Yes at 40 per cent among those certain to vote, up four points on a month ago, as undecided voters came down in favour of independence.
Backing for No remained unchanged at 54 per cent. Stripping out the undecideds, the No campaign's lead was cut to 16 points, from 58 per cent to 42 per cent.