Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels and let cool enough to handle, 5 to 10 minutes. Crumble.

Combine crumbled bacon with broccoli, sunflower seeds, and red onion in a large bowl.

Mix mayonnaise, sugar, and vinegar together in a separate bowl. Pour over broccoli mixture and mix together. Refrigerate for at least 20 minutes before serving.