Nicolaas Hendrik Christiaan de Jong Theunissen (4 May 1867 – 9 November 1929) was a South African cricketer who played in one Test match in 1889.

Described at the time as "a right-hander who bumps them down rather short, with variety of pace and a quick break back",[1] Theunissen was an opening bowler who took 34 wickets in three non-first-class matches for provincial sides against the touring R. G. Warton's XI in 1888–89.

[2] Some of the English batsmen were hampered by blows on the knuckles and the ribs from his deliveries.

[3] He was allowed to play in the Second Test at the end of the tour, when he opened South Africa's bowling but failed to take a wicket.

[5] In the second, which began the day after the previous one ended, he took 2 for 47 and 7 for 41 for Cape Town Clubs against Natal.