Ice bath before or after workout is the one cold immersion question where the timing genuinely changes the outcome, and it is the question most equipment sellers do not want to discuss. The short version: after feels better and may cost you some of the adaptation you trained for, and which matters depends on what you are training for.
The adaptation problem
Several controlled studies have found that cold water immersion immediately after resistance training reduces gains in strength and muscle size over a training block compared with the same program without it. The proposed mechanism is that cold blunts the inflammatory signalling that drives the adaptation. The finding is not universal but it is repeated enough to plan around.
When after is fine
If the next session matters more than the adaptation from this one, cold after is reasonable: a tournament day, back to back sessions, a heavy competition block, or training in heat. The reduction in perceived soreness is real and consistent, and for endurance work the evidence for blunted adaptation is much weaker than for resistance training.
Before a session
Cold before training lowers muscle temperature, which reduces power output for a while, so it is generally a poor idea immediately before anything explosive or heavy. The exception is pre-cooling before endurance work in hot conditions, where lowering core temperature first has been shown to improve performance.
The practical arrangement
Most people lifting for strength or size are best putting the plunge on a rest day, or several hours after training rather than straight afterwards. That keeps the recovery and sleep benefits people actually value while stepping around the one finding that argues against it. If you are training for general health, none of this is worth much worry.
Questions people ask about ice bath before or after workout
Does an ice bath ruin muscle gains?
Immediately after resistance training it has been shown in several studies to reduce strength and size gains over a block. Moving it to a rest day or several hours later avoids the conflict.
Is it different for running?
The evidence for blunted adaptation is much weaker for endurance work, and reduced soreness between hard sessions is a real benefit.
Should I ever plunge before training?
Not before heavy or explosive work, because cold muscle produces less power. Pre-cooling before endurance work in heat is the exception.