A typical 1 cup diced serving of watermelon contains 0.9 grams of protein (0.61 g per 100 g), which makes it only a minor source of protein. That covers roughly 2% of the 50 g reference Daily Value, so it adds toward your protein total for the day.
Only about 8% of the calories in watermelon come from protein, so it is more of an energy food than a protein food. It is worth eating for other reasons; the protein it adds to a day is incidental.
Among the 71 fruits in the database, watermelon ranks 46th by protein per serving, and 738th out of all 796 foods on the site. Set against applesauce, another fruit entry in the database, a serving carries about 0.5 g more protein (applesauce has 0.4 g per 1 cup).
It is an incomplete protein on its own; pairing it with other plant foods across the day covers the full range of essential amino acids. The serving above is measured by volume at 152 g. The per-100 g figure lets you compare it directly against any other food here, whatever its serving.
Source: USDA FoodData Central #167765 - "Watermelon, raw" (SR Legacy, published 4/1/2019). Values are the published reference figures for that record, not lab measurements for a specific brand or batch.
Quick answers
Watermelon protein FAQ
How much protein is in watermelon per 100 g?
Watermelon has 0.61 g of protein per 100 g. A typical 1 cup diced serving (152 g) provides 0.9 g.
Is watermelon a complete protein?
No. Watermelon is an incomplete protein on its own; pairing it with other protein sources across the day covers the full range of essential amino acids.
How much of the Daily Value does a serving of watermelon cover?
A 1 cup diced serving of watermelon contains 0.9 g of protein, about 2% of the 50 g reference Daily Value.
Is watermelon a good source of protein?
Watermelon is only a minor source of protein, and about 8% of its calories come from protein, which makes it more of an energy food than a protein food. Set against applesauce, another fruit entry in the database, a serving carries about 0.5 g more protein (applesauce has 0.4 g per 1 cup).