The London and North Eastern Railway (LNER) Class J39 was a class of medium powered 0-6-0 steam locomotive designed for mixed-traffic work throughout the former LNER system between London and the north of Scotland.

The class was introduced by Nigel Gresley in July 1926, based on his previous Class J38 (introduced in January 1926) but with larger driving wheels. The larger wheels enabled them to be used on both passenger and freight trains, although at the expense of a lower tractive effort. As a result they were given the BR power classification 4P/5F, rather than the 6F of the earlier class. A total of 289 examples were built over the next fifteen years, mostly built by the LNER's Darlington Works although 28 were built by Beyer, Peacock and Co. in 1935.

The larger wheels necessitated the provision of low splashers over the front two wheels, which is the main means of differentiating between the two classes. However the increased speed afforded by the larger wheels caused the big end bearings to overheat, something that was aggravated by their use on passenger trains, and reliability suffered as a result. The locomotives were all fitted with superheaters and Ross 'Pop' safety valves.

All passed into British Railways ownership in 1948 and they were numbered 64700-64988. They began to be withdrawn from service in 1959 and all examples had been scrapped by the end of 1962. No. 64747 served out the remainder of its days as a stationary boiler at the Woodford shed until October 1964.

355 locomotives were scheduled to be built, but 289 were built. Each of the LNER's three main areas received an allocation of new locomotives, as follows:

All were built at Darlington, except for the 28 ordered in October 1935, which were built by Beyer Peacock (works nos. 6802–29). Cancelled locomotives shown in parentheses.

As built the locomotives were mostly given numbers in two broad groups:

The final batch, built in 1941, took consecutive numbers 3081–98 within the former GNR series.