Cockpit said late on Tuesday that the strike planned for Thursday would also affect Lufthansa's freight subsidiary, Cargo.
Flights with the company's budget airlines Germanwings and Eurowings will not be disrupted, however.
On Monday evening, the pilots' union confirmed that short and medium-haul flight pilots would walk out between 00:01 and 23:59 local time (23:01 and 22:59 UTC) on Wednesday.
The strikes follow months of unsuccessful negotiations between Lufthansa and Cockpit over planned changes to retirement benefits for new pilots.
Lufthansa pilots are currently able to retire at age 55 and receive up to 60 percent of their pay until the normal pension kicks in at the age of 65. But the airline plans to increase the age at which pilots can take early retirement.
The German airline faced a string of similar strikes in 2014, peaking with two walkouts in the space of a week last December. The strikes wiped up to 200 million euros ($226 million) from the airline's operating profit for last year. Cockpit, a trade union for both pilots and flight engineers, has roughly 9,300 members.
ksb/bk (AFP, dpa)