Paris Hilton & Boyfriend River Viiperi -- The Model Relationship

For once, Paris Hilton let her younger model boyfriend River Viiperi take center stage ... when he walked in a fashion show in Barcelona on Monday.

The 31-year-old socialite enjoyed the view as her 21-year-old Spanish catwalker strutted his stuff on the runway for the clothing brand Mango.

From the looks of it, their relationship has legs.