In wet and windy conditions in Wales, Langer at two under par holds a one-stroke advantage over England's Carl Mason and Argentine Mauricio Molina.
Langer, chasing a 10th Senior major, was a runaway victor at the 2014 Senior Open, also staged at Porthcawl.
Langer's score of 69 puts him two shots clear of a host of players, including Colin Montgomerie and Tom Lehman.
Defending champion Paul Broadhurst shot a four-over-par round of 73.
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Bernhard Langer picked up from where he left off at Royal Porthcawl in the first round of the Senior Open.