| Jack Cornelsen | |
| Birth Date: | 13 October 1994 |
| Birth Place: | Gold Coast, Queensland, Australia |
| Height: | 1.950NaN0 |
| Weight: | 1100NaN0 |
| Relatives: | Greg Cornelsen (father) |
| Position: | Number 8, Lock |
| Currentclub: | Saitama Wild Knights |
| Years1: | 2015–2016 |
| Clubs1: | Queensland Country |
| Apps1: | 9 |
| Points1: | 762 |
| Years2: | 2018– |
| Clubs2: | Saitama Wild Knights |
| Apps2: | 54 |
| Points2: | 80 |
| Repyears1: | 2021– |
| Repteam1: | Japan |
| Repcaps1: | 16 |
| Reppoints1: | 0 |
| Repyears2: | 2023 |
| Repteam2: | Japan XV |
| Repcaps2: | 2 |
| Reppoints2: | 0 |
| Clubupdate: | 28 August 2023 |
| Repupdate: | 28 August 2023 |
is a professional rugby union player who plays as a number eight for Japan Rugby League One club Saitama Wild Knights. Born in Australia, he represents Japan at international level after qualifying on residency grounds.[1] He is the son of former Australian international player Greg Cornelsen.
Cornelsen received his first call-up to his adopted country's wider training squad in April 2021, ahead of British and Irish Lions test.[2] On 24 May, he was named in the 36-man squad for the match against the Sunwolves and tour of Scotland and Ireland.[3] Cornelsen qualified for Japan on residency in November 2020.[4]