The move comes after a rebound in Glasgow-based STV's share price in recent months.
ITV's stake dates back to the 1999 purchase by Granada, then a media and hotels giant, of an 18.6% holding in what was then called Scottish Media Group (SMG) from Mirror Group.
At the time a number of broadcasters were seeking to take over SMG, which owned The Herald.
STV told the stock market yesterday that on Monday ITV had sold all its 2.65 million shares.
With STV's shares closing at 306.5p, this could have netted the company upwards of £8m.
Yesterday, STV's shares fell 9.5p or 3.1% to 297p on the news.
Paul Richards, analyst at Numis, said: "The disposal is not material for ITV, which has held its stake since the 1990s and has been diluted by not participating in rights issues.
"Given the size of the placing and the strength of STV's shares over the past couple of quarters, it's not surprising to see the shares off today."
An ITV spokesman said: "We have held these shares for a number of years and decided that this was an appropriate time to take profit on our investment."
When Granada purchased the stake, realising a £47m profit for Mirror Group, analysts thought the deal put the Manchester company in pole position to take over SMG which was also coveted by rival ITV broadcasters Carlton and United News and Media.
Granada merged with Carlton to form ITV in 2003. But STV has remained independent. This is despite frequent speculation, including after ITV's purchase of Channel Television in 2011, that it might seek to snap up the remaining independent players STV and UTV.
This week's share sale suggests this is very unlikely.
ITV's stake was diluted by a £95.1m two for one share issue by STV in 2007 designed to pay down debt and alleviate liquidity problems as the credit crunch began.
After the cash call, STV chief executive Rob Woodward overhauled the company, selling its cinema advertising arm Pearl & Dean and Virgin Radio.
The results of this are beginning to be felt. Earlier this year STV announced it expects to make its first dividend payment for eight years in early 2014. An anticipated 1.5p a share pay-out for 2013 will be the first during Mr Woodward's six-year tenure.
Despite a slight sell-off since mid-November, STV's shares are trading at more than six times their price at their lowest.
Mr Woodward has had frosty relations with his counterparts at ITV.
A legal dispute over network programme budget contributions as STV opted out of more ITV network shows.
In April 2011 the two sides agreed an £18m settlement.
There is no suggestion that the share sale has anything to do with relations between the two companies.
The sale comes at a time of rising confidence in the UK advertising market.
Giant US advertising agency group, Interpublic, this week said UK advertising growth should hit 4.6% in 2014, up from 4.2% this year due to the World Cup and the increasing impact of social and mobile media.