Yes, you can. You have to demonstrate that you have been living together continuously for a year (In or outside of Belgium) or know each other at least two years, and then you have to proof that you have been keeping contact regularly (through phone, letters, e-mail) and that you have met each other at least three times in the last two years, for a total of at least 45 days.
Be careful: if your partner is obliged to have a visa he will have to request a visa type C for a short period of 90 days. In that period you have to make sure to make a declaration of living together legally.
You can find more information on the website of Kruispunt Migratie-Integratie or in English on the site of the Immigration Service.