Jacob Thomas "Jack" Grein (generally referred to as J. T. Grein; 11 October 1862 – 22 June 1935) was a British impresario and drama critic of Dutch origin who helped establish the modern theatre in London.

[2] Grein married the actress Alice Augusta Greeven in 1904; she later wrote and edited a biography of him under the pen name Michael Orme.

[2] The influences were not all one way: in April 1907, Grein organised a visit by Herbert Beerbohm Tree's company – based at His Majesty's Theatre – to Hanover and Berlin.

The visit was made at the personal invitation of the German Emperor; the imperial railway train transported them from the Hook of Holland.

Grein and Tree were awarded the Order of the Red Eagle for their successful tour.