The successor to the famed Hubble Space Telescope, Webb will now launch between March and June 2019 from French Guiana, following a schedule assessment of the remaining integration and test activities, NASA said.
The telescope will be the world's most powerful space telescope ever built, serving thousands of astronomers worldwide, according to the US space agency.
The 6.5-metre diameter infrared-optimised telescope is designed to study an extremely wide range of astrophysical phenomena, including the first stars and galaxies that formed, the atmospheres of nearby planets outside our solar system, and objects within our own solar system.
"The change in launch timing is not indicative of hardware or technical performance concerns," said Thomas Zurbuchen, associate administrator for NASA's Science Mission Directorate.
"Rather, the integration of the various spacecraft elements is taking longer than expected," said Zurbuchen.
As part of an international agreement with the European Space Agency (ESA) to provide a desired launch window one year prior to launch, NASA recently performed a routine schedule assessment to ensure launch preparedness and determined a launch schedule change was necessary.
The spacecraft itself, comprised of the spacecraft bus and Sun-shield, has experienced delays during its integration and testing.
The additional environmental testing time of the fully assembled observatory - the telescope and the spacecraft - will ensure that Webb will be fully tested before launching into space, NASA said.
All the rigorous tests of the telescope and the spacecraft to date show the mission is meeting its required performance levels, it said.
Existing programme budget accommodates the change in launch date, and the change will not affect planned science observations.
"Webb's spacecraft and Sun-shield are larger and more complex than most spacecraft," said Eric Smith, programme director for the James Webb Space Telescope at NASA.
"The combination of some integration activities taking longer than initially planned, such as the installation of more than 100 Sun-shield membrane release devices, has meant the integration and testing process is just taking longer," said Smith.
"Considering the investment NASA has made, and the good performance to date, we want to proceed very systematically through these tests to be ready for a Spring 2019 launch," he said.
Webb is an international project led by NASA with its partners ESA and the Canadian Space Agency.