| Unit Name: | No. 503 (County of Lincoln) Squadron RAuxAF |
| Dates: | 5 October 1926 – 1 November 1938 |
| Country: | United Kingdom |
| Branch: | Royal Auxiliary Air Force |
| Type: | Inactive |
| Role: | Bomber |
| Command Structure: | No. 6 (Auxiliary) Group RAF, Bomber Command |
| Garrison Label: | Base |
| Nickname: | County of Lincoln |
| Identification Symbol: | The arms of the county of Lincoln with an eagle above (unofficial) |
| Identification Symbol Label: | Squadron Badge heraldry |
| Identification Symbol 2 Label: | Squadron Codes |
No. 503 (City of Lincoln) Squadron RAuxAF was an auxiliary squadron of the Royal Air Force. It operated as a bomber squadron in the 1920s and 1930s, but was disbanded before the outbreak of the Second World War.
The squadron was formed on 5 October 1926 at RAF Waddington in Lincolnshire as No. 503 (Bombing) Squadron of the Special Reserve, manned by part-time volunteers. It was at first equipped with Fairey Fawn light day bombers. It re-equipped with the partly wooden Handley Page Hyderabad heavy night bomber in 1929, receiving the improved, all-metal Handley Page Hinaidi from 1933. Some squadron members thought the Hyderabad superior to the Hinaidi, but when a squadron member discovered dry rot in the longeron of a Hyderabad they settled for the 'newcomer', some almost having spent their life of 700 flying hours already.
In October 1935, it changed role again to become a day bomber squadron, receiving Westland Wallaces. These were replaced by the Hawker Hart in June 1936, these in turn starting to be replaced by the updated Hawker Hind in July 1938. However, in 1938 it was decided to switch the role of the Auxiliary Air Force to that of air defence, and on 1 November 503 Squadron was disbanded by renumbering it to No.616 (South Yorkshire) Squadron and moving it to RAF Doncaster.
| October 1926 | June 1929 | Mks.II & III | ||
| February 1929 | January 1934 | |||
| October 1933 | November 1935 | |||
| October 1935 | September 1936 | Mk.I | ||
| November 1935 | September 1936 | Westland Wallace | Mk.II | |
| June 1936 | November 1938 | |||
| June 1938 | November 1938 |
| October 1926 | November 1938 |
| November 1926 | S/Ldr. R.D. Oxland | ||
| 15 February 1930 | W/Cdr. The Hon. L.J.E. Twistleton-Wykeham-Fiennes | ||
| 15 February 1930 | 11 May 1931 | W/Cdr. H.P. Lale, DSO, DFC | |
| 11 May 1931 | 9 August 1933 | W/Cdr. H.I. Hanmer, DFC | |
| 9 August 1933 | January 1936 | W/Cdr. A.P.V. Daly, AFC | |
| January 1936 | S/Ldr. A.F. James | ||
| 30 May 1937 | S/Ldr. H. Power (acting) |