This is true. A rectangle is a quadrilateral with 4 right angles, meaning each pair of adjacent sides meet at a right angle. From the father of geometry himself:
“If a line segment intersects two straight lines forming two interior angles on the same side that sum to less than two right angles, then the two lines, if extended indefinitely, meet on that side on which the angles sum to less than two right angles.”
The other side to this, of course, is that if you have a line segment intersecting two straight lines, and you *do* get two interior right angles, those straight lines will never meet, which is exactly what it means to be parallel. If you pick any three line segments on a rectangle, you get exactly this behavior, meaning each pair of line segments on a rectangle is parallel.
So yes, a rectangle *is* a parallelogram.