Can one defendant force another defendant to pay their share of damages in Ontario?

Yes. The Negligence Act allows a defendant who ends up paying a judgment, or more than their fair share of it, to seek contribution and indemnity from other parties who were also at fault, based on each party's respective degree of responsibility as determined by the court. This means the plaintiff's right to collect the full judgment from any one defendant, discussed in relation to joint and several liability, does not leave that defendant stuck bearing the whole loss alone if others were also to blame.

In practice, defendants often do not wait until after a judgment to sort this out; they commonly bring crossclaims against each other, or third-party claims against parties not originally named by the plaintiff, within the same lawsuit, so that the court can apportion fault among everyone involved in a single proceeding rather than requiring a separate later lawsuit. This is generally more efficient for everyone, including the plaintiff, since it resolves the full picture of who was responsible for what at once. Determining the right apportionment between defendants is a fact-specific exercise based on each party's actual conduct, not simply an equal split among however many defendants are involved.