Can you counter a spell that has split second with a triggered ability that counters spells?

Split second (CR 702.61) prevents players from casting spells or activating abilities (other than mana abilities) while the split second spell is on the stack. However, it explicitly does not stop triggered abilities from triggering or resolving.

CR 702.61b states: 'While a spell with split second is on the stack, players can't cast other spells or activate abilities that aren't mana abilities. Triggered abilities can still trigger.' This means if a triggered ability happens to counter spells — for example, one that says 'whenever a spell is cast, counter it' — that trigger will still go on the stack and can resolve normally.

The key distinction is the source of the effect. Players are locked out of taking actions (casting, activating), but the game itself still processes triggered abilities automatically. No player 'activates' a triggered ability; it simply triggers.

Concrete example: Your opponent casts Krosan Grip (which has split second) targeting your artifact. You control a permanent with the triggered ability 'Whenever an opponent casts a spell, counter that spell.' That trigger fires when Krosan Grip is cast, goes on the stack on top of it, and can resolve to counter Krosan Grip — even though split second is in effect.

So if you have an already-existing triggered ability that counters spells, split second will not protect the spell from it. Split second only shuts down new spells and activated abilities during its window on the stack.