Does a spell that says 'counter target spell' work on a spell with split second?

Split second (CR 702.61) means that as long as a spell with split second is on the stack, players cannot cast other spells or activate abilities that aren't mana abilities. This effectively 'locks' the stack for the duration of that spell's resolution window, preventing opponents from responding with counterspells.

A 'counter target spell' effect is delivered via a spell (e.g., Cancel, Counterspell). Since casting that counterspell in response to a split second spell is prohibited by CR 702.61a, the counterspell simply cannot be cast at that moment. You cannot target the split second spell with a counterspell because you can't cast the counterspell at all while split second applies.

It is important to note that split second does NOT stop triggered abilities from triggering and going on the stack (CR 702.61b). If a permanent you control has a triggered ability that would counter spells, and that trigger fires, it can still go on the stack. But an activated ability or a spell that counters cannot be used in response.

Concrete example: Your opponent casts Sudden Death (a card with split second). You want to cast Counterspell to stop it. You cannot — CR 702.61a prohibits you from casting any spell while Sudden Death is on the stack. Sudden Death resolves unimpeded.

So the answer is definitively No: a 'counter target spell' spell cannot be cast in response to a split second spell, making split second one of the game's strongest forms of stack protection.