Peter Thomas Kirstein (né Kirschstein; 20 June 1933 – 8 January 2020) was a British computer scientist who played a role in the creation of the Internet.

He made the first internetworking connection on the ARPANET in 1973, by providing a link to British academic networks, and was instrumental in defining and implementing TCP/IP alongside Vint Cerf and Bob Kahn.

[9][10] Building on the work of Donald Davies at the National Physical Laboratory in the 1960s,[11] in 1973 Kirstein's research group at University College London became one of only the two international connections on the ARPANET, alongside Norway (NORSAR and NDRE).

[20] He chaired the International Cooperation Board (ICB), formed by Cerf in 1979, to coordinate activities to develop packet satellite research.

[24] He led the Silk Project, which provides satellite-based Internet access to the Newly Independent States in the Southern Caucasus and Central Asia.

Shortly after his death, Steve Hailes, Head of Department for UCL Computer Science, wrote about him: "Peter was very widely recognised as a pioneer of the Internet and has many honours to his name [...] Much of this was undoubtedly down to an incredibly logical mind, coupled with a level of interest, vision and determination that saw him retire only late last year at the age of 86. [...]

Peter was also deeply empathetic and sensitive: he was both gentleman and a gentle man, he was a source of encouragement and sage advice, he was persuasive, open-minded, fair and never afraid to learn something new or to admit that he didn't know.