I thank thee

[Caballero]

¿Cómo se dice en castellano?

[Luna Azul]

Te agradezco.

Since it's old English, it would be "Os agradezco"

[wrholt]

¿Cómo se dice en castellano?

thou/thee/thy/thine = tú/te/ti/tu/tuyo or vos/te/vos/tu/tuyo

So, "te agradezco", perhaps.

[aleCcowaN]

"Te agradezco"

Thou = tú

(English lost that pronoun, Spanish didn't)

If you just want to sound Renaissance style, you may use "vos agradezco" or "os agradezco" (the last one is voseo reverencial when a single person is addressed, in such case it has a similar ancient aura as "I thank thee")

[Caballero]

Now I hear this all the time on tv.