| Honorific-Prefix: | The Right Honourable |
| The Lord Williamson of Horton | |
| Honorific-Suffix: | GCMG CB PC |
| Office: | Convener of the Crossbench Peers |
| Term Start: | 2004 |
| Term End: | 2007 |
| Predecessor: | The Lord Craig of Radley |
| Successor: | The Baroness D'Souza |
| Office3: | Member of the House of Lords Lord Temporal |
| Term Start3: | 5 February 1999 |
| Term End3: | 30 August 2015 Life peerage |
| Birth Date: | 8 May 1934 |
| Alma Mater: | Exeter College, Oxford |
| Spouse: | Patricia Smith (m. 1961) |
| Children: | 2 |
David Francis Williamson, Baron Williamson of Horton (8 May 1934 – 30 August 2015) was a senior British and European civil servant, as well as a member of the House of Lords.
Williamson was educated at Tonbridge School and Exeter College, Oxford. He served in the Royal Signals 1956–58 as his national service. He married Patricia Smith in 1961; they had two sons.
He began his civil service career in 1958 at the Ministry of Agriculture, Fisheries and Food, becoming Principal Private Secretary to the minister in 1967.
He was Deputy Director-General for Agriculture in the European Commission from 1977 to 1983 and Secretary-General of the European Commission from 1987 to 1997. From 1983 until 1987, Williamson had returned to the United Kingdom to serve as Deputy Secretary and head the European Secretariat in the Cabinet Office.
After leaving Brussels, Williamson was created a life peer on 5 February 1999 with the title Baron Williamson of Horton, of Horton in the County of Somerset, and sat as a crossbencher in the House of Lords. Lord Williamson of Horton was the convenor of the crossbenchers, a task that involves keeping the other non-aligned members up-to-date with the business of the House.
As well as being appointed a Companion of the Order of the Bath (CB) in 1984 and a Knight Grand Cross of the Order of St Michael and St George (GCMG) in 1998, Williamson was awarded the Knight Commander's Cross of the Bundesverdienstkreuz by Germany, the Commander Grand Cross of the Royal Order of the Polar Star by Sweden and made Commander of the Légion d'honneur by France. He was also sworn of the Privy Council in April 2007.