Do both parents have to consent before a child's surname can be legally changed in Ontario?

Generally, yes — under the Change of Name Act, changing a child's surname normally requires the consent of everyone who has decision-making responsibility or parenting time for the child, which usually means both parents where they share those responsibilities. If one parent won't consent, the parent seeking the change can't simply proceed without them; the options are to try to resolve it directly, or to ask a court to dispense with the other parent's consent or to decide the name change as part of a parenting matter.

A court asked to override a parent's objection will apply the best-interests-of-the-child test rather than simply deciding based on which parent applied first — the child's identity, relationship with each parent, and any practical reasons for the change, such as safety or an existing established name in daily use, are all relevant. Because a straightforward government application depends on having everyone's consent, a parent facing an objection from the other parent should expect to need a court process rather than a simple form, and should get legal advice on what evidence supports the change before starting.