The tent has been erected on the raised walkway beside the pedestrian entrance to the emergency department.
The queue to get into the department often stretches outside the building, and while there is a small permanent canopy, the new tent provides additional shelter.
A convex mirror allows staff to monitor the queue from the doors of the hospital.
Hospital bosses have now applied to Lambeth Council for temporary planning permission to retain the open-sided tent for a longer period.
Plans submitted to the council show the addition of a 'traffic light system' so that those in the queue can see when space becomes available inside.
Floor markings indicated on the plans show that much of the space under the marquee will be designated for wheelchair users.
In his most recent report to the Guy's and St Thomas' board, chief executive Ian Abbs wrote: "The trust has ... continued to work hard to manage an increasing number of patients who are accessing our urgent and emergency care services.
"Prior to December, our emergency department continued to experience a high level of activity across all patient types. At times, daily attendances for the month regularly exceeded 550 per day. In total, the trust recorded over 17,800 attendances for the month; our second busiest month since the start of the pandemic."
A report from Guy's and St Thomas' submitted to a Southwark Council scrutiny commission this week said that "emergency department attendances are lower than we've seen in the summer and autumn months of 2021/22, but is a significant increase [on] the volume of activity reported a year ago.
"Despite operational pressures, we have been able to consistently see above 80 per cent of unscheduled emergency care attendances within 4 hours."
The emergency department at St Thomas' was reopened by the Princess Royal in 2018 after a £20 million revamp. At that time the hospital was seeing around 420 emergency patients a day.
• For more information see planning application 22/00679/FUL