Thomas Penfield Jackson (January 10, 1937 - June 15, 2013) was a judge for the United States District Court for the District of Columbia.

Jackson was born in Washington, D.C. He graduated from Dartmouth College with an A.B. in the class of 1958, and from Harvard Law School with a LL.B. in 1964. He served in the United States Navy from 1958 to 1961. He was appointed to the Court in 1982 after serving as president of the District of Columbia Bar Association, filling the seat vacated by Oliver Gasch. He assumed senior status in 2002 and retired in 2004. He is currently an attorney with the Jackson and Campbell, P.C., law firm.

He is perhaps best known to the public as the presiding judge in the United States v. Microsoft case. Jackson was the first in a series of judges worldwide to determine that Microsoft abuses its market position and monopoly power in ways that are highly detrimental to innovation in the industry and consumers of the products. The summary paragraph in his findings of fact is quoted below.

Most harmful of all is the message that Microsoft's actions have conveyed to every enterprise with the potential to innovate in the computer industry. Through its conduct toward Netscape, IBM, Compaq, Intel, and others, Microsoft has demonstrated that it will use its prodigious market power and immense profits to harm any firm that insists on pursuing initiatives that could intensify competition against one of Microsoft's core products. Microsoft's past success in hurting such companies and stifling innovation deters investment in technologies and businesses that exhibit the potential to threaten Microsoft. The ultimate result is that some innovations that would truly benefit consumers never occur for the sole reason that they do not coincide with Microsoft's self-interest.

Microsoft attempted to show that the judge's conduct during the case demonstrated that he unfairly favored the prosecution, but they failed to do so in court proceedings. He did speak to a reporter off the record after the evidence in the case had been heard but prior to issuing his "Conclusions of law" and this was contrary to judicial rules. Speaking with that reporter he expressed unfavorable opinions and statements about Microsoft Corporation and its employees which he had developed as a result of hearing the evidence and witnesses in the trial. "Microsoft PressPass - Microsoft's Petition for Certiorari" cyber.law.harvard.edu]</ref> Speaking about Microsoft executives, he compared them to "stubborn mules who should be walloped with a two-by-four" and "gangland killers", referring to a murder case he presided over four years earlier:

On the day of the sentencing, the gang members maintained that they had done nothing wrong, saying that the whole case was a conspiracy by the white power structure to destroy them. I am now under no illusions that miscreants will realize that other parts of society view them that way."

The judge also characterized Microsoft leader and co-founder Bill Gates as a Napoleon, "unethical", as well as comparing him to a "drug trafficker" repeatedly caught as a result of telephone wiretaps. However, it was private meetings with journalists released after the verdict but during the appeal that granted the appeal.

Judge Jackson's order that Microsoft be divided into two companies, one owning the Windows operating system and the other owning Microsoft's various application software products, including the Internet Explorer web browser, was also reversed on appeal. Eight of his factual findings about Microsoft's monopolistic practices against the Sherman Antitrust Act, however, were upheld, though cut down based on the June 2001 appeal.

Judge Jackson, in spite of the findings of the appellate court, continued to deny that any such bias existed and insisted that any perception of bias in the minds of observers was created by Microsoft. His statements were a response to several evasive tactics Microsoft used at the trial, including falsifying video evidence, non-responsiveness on the stand, and denying allegations contained in evidence. The recusal has been called into question by some commentators, as other cases have warranted a "slap on the wrist", such as the Marion Barry trial in 1990 where Jackson said, "that he had never seen a stronger government case and was upset that some jurors would not vote to convict." The appeals court upheld the trial, but dissented that "It is worth noting that the district court judge could have recused himself in this case.... The recusal option was a compelling one."

When an unrelated case involving Microsoft and charges of discrimination was assigned to him in 2001, Jackson recused himself from the case.

Jackson was also the presiding judge in United States v. Marion S. Barry, where D.C. Mayor Marion Barry was convicted of one count of cocaine possession.

Jackson died of cancer at his home in Compton, Maryland on June 15, 2013.