The probe focused on whether any doctors who treated the co-pilot, Andreas Lubitz, had been criminally negligent in not reporting him to authorities before the plane came down in France, killing 144 passengers and six crew, mostly from Spain and Germany.
"The investigation has shown no sufficient evidence of guilt by anyone still alive in connection with the Germanwings crash," a spokesman for the Duesseldorf public prosecutor's office, Christoph Kumpa, told AFP.
Lubitz, 27, deliberately flew the Germanwings plane into a French mountainside in a tragedy that raised questions about aviation safety and doctor-patient confidentiality.
Kumpa said the probe had determined that Lubitz's doctors knew he was "suffering psychologically" in the months before the disaster but that he had not been diagnosed as clinically depressed.
Rather, the investigators found, "the co-pilot did not tell the doctors treating him or anyone else in his personal life about his suicidal thoughts so that none of these people could have been expected to tell his employer or the authorities".
The probe also concluded that Germanwings had "no knowledge of psychological ailments" suffered by Lubitz.
French investigators have been carrying out their own manslaughter probe over the crash, and relatives of victims have filed a lawsuit against the Lufthansa-owned flight school that trained Lubitz.
The co-pilot was permitted to continue flying despite having been seen by doctors dozens of times in the years preceding the crash.
Lubitz was terrified of losing his sight and consulted 41 different doctors in the previous five years, including psychiatrists as well as ear, throat and nose specialists.
Following the crash, the European Aviation Safety Agency recommended more medical testing for pilots, including more psychological tests and drug and alcohol screening.
Germany's doctors' association has criticised Germanwings parent company Lufthansa and aviation regulators for failing to keep Lubitz from flying, saying that medical controls focused largely on "physical findings and laboratory tests" but neglected psychological examinations.