Am I responsible for topping up a pension shortfall the seller left behind?

In a share purchase, generally yes — if the corporation sponsors a pension plan with an underfunded shortfall, that obligation belongs to the corporation, and since you now own it, responsibility for addressing the shortfall continues with the business, subject to whatever the specific plan's own funding rules require and any protections you negotiated in the purchase agreement, such as a price adjustment or holdback tied to the shortfall.

In an asset purchase, this depends heavily on whether you're taking on the existing plan itself or setting up a new one. If you don't assume the plan, the shortfall generally stays with the seller as plan sponsor; if you do assume it, or continue it in some form, the funding obligation can come with it. This is a genuinely technical, plan-specific, and often actuarial question rather than a general rule that applies the same way to every deal.

Get the plan's actual funding status assessed by a pension specialist before closing, not just a general assurance that "it's fine," and make sure the purchase agreement reflects what you're actually agreeing to take on. A Treadstone business lawyer can help coordinate this review.