Start by turning on your Instant Pot and selecting the "Sauté" function. Add the ground turkey sausage to the pot. Use your wooden spoon to break it up as it cooks. Brown the sausage for about 5 minutes until it's fully cooked.
Once the sausage is browned, pour in the 4 cups of water, followed by the whole-wheat spaghetti. Make sure the spaghetti is submerged in the water. Then, add the marinara sauce and diced tomatoes on top. Do not stir at this stage; just layer the ingredients.
Close the lid of the Instant Pot and make sure the valve is set to the sealing position. Select the "Manual" or "Pressure Cook" setting and set the timer for 8 minutes.
When the cooking time is up, carefully perform a quick release by turning the valve to venting. Once the steam has completely escaped, open the lid.
Give your spaghetti a good stir to combine all the ingredients. If desired, you can let it sit for a few minutes to thicken up, then serve it warm. Enjoy your Instant Pot Skinny Spaghetti!