The Lothians MSP called for an overhaul of the defamation law in Scotland after the judgement, insisting the case should never have come to court.
The action had been brought by Wildcat Haven Enterprises over comments made in blogs and social media posts by Wightman in 2015 and 2016, including references to the firm being associated with a "tax haven" which it rejected.
Mr Wightman also raised concerns about the firm, established with the aim of helping to protect wildcat numbers in Scotland, offering donors an opportunity to buy a one square foot plot of land in its “wildcat haven” area in the north of Scotland. The legal status of this was challenged by the MSP.
Wildcat Haven insisted it was made clear that these would be "souvenir plots" and donors would have been aware they would not be registered owner of the land.
But Lord Clarke's judgement states: "The defamation claims made by the pursuer fail because the meanings alleged are not made out, or available defences apply.
"The pursuer’s claim for damages (in the sum of £750,000) must fail."
Mr Wightman did make four factual errors in his statements about the firm, the judge added.
But the he adds: "For the defence of fair comment to be available, it is not necessary that every fact founded upon is true."
The MSP raised more £170,000 from crowdfunding supporters to be able to challenge the legal action at the Court of Session.
He said afterwards he was "delighted" with the judgement.
"I have maintained throughout that I did not defame the pursuer and that this action should never have been brought against me," he added.
"It is vital that Parliament modernises the law of defamation to ensure that the law provides the right balance between freedom of expression and the rights of people not to have their reputations tarnished. It is also important that the law is clear, so that writers and journalists can write confidently and provide the freedom of expression that is so important in any democracy."