The paintings, dating back to the 4th century AD, were part of a larger fresco that also included the head-and-shoulders depictions of the apostles Peter and Paul, reported the Pontifical Commission for Sacred Archeology at a press conference.
The pictures of the apostles Andrew and John were not visible, however, until new laser technology was used to carefully burn off some the thick deposits of white calcium that buried them. The painting of Paul, also the oldest known, was discovered last year. Peter's, meanwhile, is not believed to be the oldest.
About 50 centimeters (19.7 inches) in diameter, the four circular portraits were located on the ceiling of a noblewoman’s burial place in the catacombs of St. Tecla.
Together covering an area of about two meters by two meters (6.6 feet by 6.6 feet), the frescoes also depict the noblewoman and her daughter.
The Vatican, which maintains the catacombs, reportedly spent about 60,000 euros (about $74,000) on the two-year archaeological project that uncovered the images.
Barbara Mazzei, who was in charge of the restoration project, told reporters the latest discovery was a "very, very emotional" one.
Experts believe the frescoes may have set the standard for future representations of the apostles.