| Dema | |
| Fullname: | William de Mattia |
| Birth Date: | 28 April 1983 |
| Birth Place: | Criciúma, Brazil |
| Position: | Midfielder |
| Currentclub: | Brasil de Pelotas (head coach) |
| Years1: | 2002 |
| Clubs1: | Tubarão |
| Years2: | 2003 |
| Clubs2: | Gama |
| Years3: | 2003 |
| Clubs3: | Tubarão |
| Years4: | 2004–2005 |
| Years5: | 2005 |
| Years6: | 2005–2007 |
| Clubs6: | Skoda Xanthi |
| Caps6: | 12 |
| Goals6: | 1 |
| Years7: | 2006–2007 |
| Clubs7: | → Kastoria (loan) |
| Years8: | 2008 |
| Caps8: | 1 |
| Goals8: | 0 |
| Years9: | 2009 |
| Years10: | 2009–2011 |
| Clubs10: | FC PoPa |
| Caps10: | 24 |
| Goals10: | 6 |
| Years11: | 2010 |
| Clubs11: | → Uberlândia (loan) |
| Caps11: | 7 |
| Goals11: | 1 |
| Years12: | 2010 |
| Clubs12: | → Tampere United (loan) |
| Caps12: | 15 |
| Goals12: | 2 |
| Years13: | 2010 |
| Clubs13: | → HJK Helsinki (loan) |
| Caps13: | 5 |
| Goals13: | 0 |
| Years14: | 2011 |
| Clubs14: | → Haka (loan) |
| Caps14: | 6 |
| Goals14: | 1 |
| Years15: | 2012–2013 |
| Clubs15: | Haka |
| Caps15: | 15 |
| Goals15: | 4 |
| Years16: | 2014 |
| Clubs16: | MyPa |
| Caps16: | 28 |
| Goals16: | 1 |
| Years17: | 2015 |
| Clubs17: | Juventude |
| Caps17: | 8 |
| Goals17: | 0 |
| Years18: | 2016 |
| Clubs18: | Guarani de Palhoça |
| Caps18: | 15 |
| Goals18: | 0 |
| Years19: | 2016 |
| Caps19: | 6 |
| Goals19: | 0 |
| Years20: | 2017 |
| Clubs20: | São Paulo-RS |
| Caps20: | 10 |
| Goals20: | 0 |
| Manageryears1: | 2019–2021 |
| Managerclubs1: | Nova Mutum |
| Manageryears2: | 2022 |
| Managerclubs2: | Grêmio Anápolis |
| Manageryears3: | 2022 |
| Managerclubs3: | Juventus Jaraguá |
| Manageryears4: | 2023–2024 |
| Managerclubs4: | Treze |
| Manageryears5: | 2025– |
| Managerclubs5: | Brasil de Pelotas |
William de Mattia (born 28 April 1983), commonly known as Dema, is a Brazilian football coach and former player who played as a midfielder. He is the current head coach of Brasil de Pelotas.