The new timetable is designed to ensure reliability for customers and cater to key workers, school pupils and those who cannot work from home.
It will come into effect from January 17.
The rail operator said the emergence of the Omicron variant has "necessitated the need for a new temporary timetable" after widespread staff shortages across its network having led to "unavoidable short-notice cancellations".
Due to Plan B measures, which has seen the Government advise people to work from home where possible, SWR said the revised timetable will sufficiently meet current demand.
Rigorous planning has gone into developing this timetable to deliver the right level of service for the current situation but SWR will continue to monitor its services to ensure that they continue to deliver for its customers.
These temporary timetable changes will remain under review going forward.
SWR’s managing director Claire Mann said: “The spread of the Omicron variant has had a significant impact on our railway, with fewer people using the train and staff shortages impacting on our ability to consistently deliver the current timetable.
“Having assessed demand and spoken to our industry colleagues, we believe this new timetable is the most effective means of ensuring our customers receive a reliable service, with short-notice cancellations minimised.
“Journey planners will be updated on a week-by-week basis, with updates taking place towards the end of each week. Customers should check their journey as close to their time of travel as possible.”