Polenta and masa harina are both types of ground corn that can be prepared into versatile doughs. The corn used to make masa harina is nixtamalized; that is, mixed with an alkaline ingredient such as ash or limestone. This process changes the flavor and also makes the nutrients more available. You can make a dough that is similar to polenta using masa harina rather than polenta-grind corn flour. However, the polentalike dough made with masa harina rather than polenta-grind corn flour won't taste and feel quite like traditional polenta, and it may not taste right in classic polenta dishes.
The processes for preparing polenta and masa harina are similar. Both methods involve mixing warm or hot water with corn flour to make a paste that thickens and solidifies as it rests. The water for traditional polenta is typically brought to a boil before you add the polenta flour, which should be stirred or whisked almost constantly until it thickens. Masa harina can be added to either boiling water or to warm water that has not been brought to a boil.
Both polenta and masa harina form lighter or denser doughs based on how much water you use relative to the amount of corn flour that you add. You can make polenta using 3 cups water to each cup of corn flour, or you can make fluffier polenta using 4 parts water to 1 cup of corn flour. Depending on whether you are making tortillas or tamales, you can use anywhere from half a cup to 2 cups of water for each cup of masa harina flour.
Dough made from polenta-grind corn flour is similar to dough made from masa harina in that its viscosity increases as it sits. Both types of dough will start out at a consistency between a liquid and a paste, depending on how much water you use. Both polenta-grind corn flour and masa harina come in coarse and fine grinds, making dough that is either grainy or smooth depending on the fineness of the grind.
The flavor of masa harina is somewhat different from the flavor of polenta-grind corn flour. Corn flour that has not been nixtamalized has a simpler, sweeter flavor while nixtamalized masa harina has a deeper, more complex flavor. If you are making a traditional Italian dish with tomatoes and basil, the flavor of masa harina may be discordant. If you are making an improvised casserole that isn't inspired by a particular culinary tradition, you can use either polenta or masa harina for a bottom or top layer.