Is the Trisagion a Catholic prayer?

[Suikoden2]

Can a Protestant pray this prayer?
Holy God, Holy Mighty One, Holy Immortal One, Have mercy on us.

 

[GreekOrthodox]

It's Eastern Orthodox, not Catholic. According to tradition (and Wikipedia )

And of course feel free to use it

The tradition recounts that during the reign of Theodosius II (408-450) Constantinople was shaken by a violent earthquake, 24 September, and that whilst the people, the emperor and the Patriarch Proclus of Constantinople (434-446) were praying for heavenly assistance, a child was suddenly lifted into midair, to whom all cried out Kyrie eleison ('Lord, have mercy'). The child was then seen to descend again to the earth, and in a loud voice he exhorted the people to pray : 'Holy God, Holy Mighty, Holy Immortal'. After giving this exhortation, the child died.

Trisagion - Wikipedia

 

[anna ~ grace]

If it is pre-Schism, it is both. Or all. The Eastern Orthodox use it often, the Oriental Orthodox use it, too, and a variation of it features in the Catholic Divine Mercy Chaplet.

Yes, you can use it. It's an awesome prayer.

 

[Albion]

Can a Protestant pray this prayer?
Holy God, Holy Mighty One, Holy Immortal One, Have mercy on us.

Absolutely. In fact, I know some Protestant churches that recommend it.