| Honorific Prefix: | Dato Paduka |
| Mahadi Wasli | |
| Honorific Suffix: | DPMB |
| Office: | Ambassador of Brunei to Vietnam |
| Term Start: | 20 July 2007 |
| Term End: | 29 August 2013 |
| Predecessor: | Mahdi Abdul Rahman |
| Successor: | Sahari Salleh |
| Office1: | Permanent Representative of Brunei to the United Nations in Geneva |
| Term Start1: | 28 August 2003 |
| Term End1: | July 2007 |
| Predecessor1: | Hamid Jaafar |
| Successor1: | Alaihuddin Taha |
| Office2: | Ambassador of Brunei to Russia |
| Term Start2: | 27 August 2001 |
| Term End2: | 2003 |
| Successor2: | Janin Erih |
| Office3: | Ambassador of Brunei to Germany |
| Term Start3: | 1997 |
| Term End3: | 2001 |
| Predecessor3: | Pengiran Maidin |
| Successor3: | Adnan Buntar |
| Birth Place: | Brunei |
| Occupation: | Diplomat |
| Alma Mater: | MARA Institute of Technology |
Mahadi bin Haji Wasli is a Brunei diplomat who became the Deputy Secretary-General of ASEAN from 1994 to 1997, ambassador to Russia from 2001 to 2003, Vietnam from 2007 to 2013, and the permanent representative to the United Nations (UN) in Geneva from 2003 to 2007. Notably, he also held non-resident ambassadorship to Ukraine from 2004 to 2007, and Switzerland.
From 1994 to 1997, Mahadi held the position of Deputy Secretary-General of the Association of Southeast Asian Nations (ASEAN).[1] Following this, from 1997 to 2001, he served as the ambassador of Brunei to Germany, with a residence in Bonn and later in Berlin.[2] From 1999 to 2001, he concurrently served as the permanent representative of Brunei to the United Nations in Geneva, and holding the title of non-resident ambassador to Switzerland.[3] He served as the Brunei ambassador to the Russian Federation from 2001 to 2007.[4] Additionally, from 2004 to 2007, he served as the non-resident ambassador of Brunei to Ukraine, playing a crucial role in facilitating the visit of Sultan Hassanal Bolkiah to Ukraine in 2004.[5]
From 10 July 2007 to 29 August 2013, he extended his diplomatic service as the Brunei ambassador to Vietnam.[6] [7] On 8 August 2010, he presented his letter of credential to President Nguyễn Minh Triết.[8] Together with Mustappa Sirat, they attended the first ASEAN Defence Minister's Meeting – Plus (ADMM+) on 12 October 2010 in Hanoi.[9] Later on 10 December 2020, General Ngô Xuân Lịch gave him the chairmanship of the ADMM and ADMM+.[10]
He has earned the following honours;