Last week, England batting star Pietersen took to Twitter to question the credentials of Knight, who averaged just 23.96 in 17 Tests with one hundred, as a Test match commentator for Sky Sports.
"Can someone please tell me how Nick Knight has worked his way into the commentary box for the Tests? Ridiculous," said Pietersen, who has 19 Test hundreds to his credit.
The ECB held a disciplinary hearing with managing director Hugh Morris and team director Andy Flower present, and opted to impose an undisclosed fine on Pietersen, part suspended for 12 months.
Pietersen's comments about Knight were especially sensitive, as Sky agreed in January a new four-year deal to show live international and county cricket believed to be worth at least ?260 million -- the ECB's main source of income.
An ECB statement statement said: "Kevin Pietersen attended a disciplinary hearing convened by Hugh Morris, managing director of England cricket and Andy Flower, England team director.
"The hearing considered recent comments made by Pietersen on Twitter to be prejudicial to the interests of the ECB and a breach of the England player conditions of employment in relation to clauses regarding public statements.
"Pietersen has been fined an undisclosed sum, part of which has been suspended for 12 months, in accordance with the terms and conditions of his England central contract."
Morris, himself a former international batsman, is wary of Twitter, once labelling its use by England players as "like giving a machine gun to a monkey".
Pietersen had previously run into trouble with the England management regarding Twitter after using the site to announce he'd been dropped from a one-day squad.
But Pietersen, speaking before this disciplinary hearing, said Tuesday he would not stop using Twitter.
"No, not at all, no. I won't stop tweeting, no."
The second Test between England and the West Indies at Trent Bridge -- in which Pietersen is expected to play -- starts on Friday with England 1-0 up in the three-match series following a five-wicket win at Lord's concluded Monday.