In which latter-day temple were baptisms for the dead first performed?

The first baptisms for the dead in this dispensation were performed in the Mississippi River near Nauvoo. In November of 1841 the font in the unfinished Nauvoo Temple was dedicated for this purpose, and the first baptisms for the dead in a temple were performed in that font.  For further details, see the article on "Baptism for the Dead" in the Encyclopedia of Mormonism: contentdm.lib.byu.edu

When I used to visit Nauvoo as a boy long before the construction of the current temple, the temple lot was just a depression in the ground with various stones lying around.  One of the principal identifiable features at that time was the place where the original baptismal font had stood. -- Kevin Barney