Piraeus is a safe city in general. Avoid walking at might close to the port area, train & metro at the port area. You should mind your personal belongings while you are in the train/metro and the port. It is safe to walk around the center and the marina area.

In Athens and Piraeus, I feel calm walking alone. I haven't had any problems and I hope everything continues to be this good, although it's always important to stay alert! I love the Greeks, they are very loving and kind, always willing to help.