Fill a cocktail shaker with ice cubes to chill the ingredients properly.

Pour the vodka, grapefruit juice, lime juice, and triple sec into the shaker.

Secure the lid on the shaker and shake vigorously for about 15 seconds to combine and chill the mixture.

Strain the cocktail into a chilled martini glass.

Garnish with a slice of grapefruit on the rim of the glass and serve immediately.