No, eating excess protein alone does not make you fat. However, consuming more calories than you are burning by trying to increase your protein intake may result in weight gain.
Extra Calories = Fat
Anytime you are consuming more calories than you burn the excess is stored as fat in the body for energy later down the road. Relatively speaking, compared to fats and carbohydrates, protein has come out with the best reputation for keeping a slim figure. There have been multiple studies conducted that seem to show that increasing the recommended daily protein intake by over 3 times will still not lead to fat gain. In fact, when protein is consumed over the recommended daily intake, people benefited from the increase in protein.
If the excess protein is not being used for fat, then where does it go; your liver ships out the amino acids to use for muscle repair, energy or fat. Excess amino acids are most likely excreted out of the body. If you can keep the calories down and increase protein, you will not be gaining weight. People notice weight gain and associate it with the increase in protein when they start consuming more fish, meats, cheeses, etc. and their daily caloric intake can be blamed for the weight gain. Every gram of protein has about 4 calories so if you had a protein shake with 20 grams of protein and no other ingredients you would be at 80 calories.
Eating the Right Amount
To reduce risk of gaining weight from consuming too many protein calories, stick to full meals with a balance of veggies and protein. If you drink all your calories, you typically are not satisfied as when you eat a protein rich meal with additional fiber. Also, it is important to vary the types of protein that you consume to ensure that you are getting all the essential amino acids that the human body doesn’t naturally produce. If you stick with a high protein diet and limit your calories to what you burn, you will not gain weight from excess protein.
References: MensFitness, WomensHealthMag