If you marry someone that has a child, and the biological father is paying child support for that child, and you get a divorce after 4 years, are you responsible to pay support for that child if you never filed to adopt that child?

Generally not.

Sorry I should also say we are in Canada.

In Canada there is a chance that you would pay child support as a suplemental to what the bio dad pays. Consult your local barrister to be sure.

Yes in Canada there are laws that make you pay child support as a supplement to what the biological father pays. Thanks for your help!

Canadian law and court practices are very inconsistent and bizarre in that matter.
It is impossible child support to be paid by former step-parents in Quebec , New Brunswick and Nova Scotia,for example.
Let's see what is in BC:
Under the Divorce Act, a person ending a marriage may be required to pay support for a child of the marriage.This phrase is defined broadly and includes children who are:
1. A child of both of the spouses;
2. A child for whom both the spouses stand in the place of a parent; or
3. A child for whom one spouse is a parent and the other spouse stands in the place of a parent.

In determining whether a person “stands in the place of a parent” the Court will look at the following factors:
1. Whether the child participates in the family as would a biological child;
2. Whether the person provides financially for the child;
3. Whether the person disciplines the child as a parent;
4. Whether the person represents to the child, the family, the world, either explicitly or implicitly, that the person is a parent to the child; and
5. The nature or existence of the child's relationship with the absent biological parent.


There is a threshold test,too.


The Threshold Test
When you marry someone who has children, you automatically become a step-parent to those children, but you do not automatically stand in the place of a parent for them. This requires something more than simply being married to the child's parent. You must have crossed a threshold and acted in a way that showed you intended to be like a parent to that child. A judge looks at many factors to see if you have crossed that threshold. The judge will consider the following:
* Whether you provide financially for the child
* Whether you discipline the child as a parent would
* Whether you hold yourself out to the child and to the public that you are responsible as a parent to that child
* Whether the child still has a relationship with her absent biological parent
* Whether you and your spouse have had your own child, creating a blended stepfamily
* Whether your step-child participates in your family the same way as your biological child does (or would, if you had one)
* Whether the step-child uses your surname
* Whether you have ever considered adopting the child
* Whether the step-child calls you Dad or Mom
* Whether you have a good or a poor relationship with your step-child, and how long it lasted
* Whether you ever took any steps to exclude the biological parent from the child's life and showed you intended to replace that parent
* How old the child is, and if she ever knew her absent biological parent

The obligations of the stepparent are not secondary to those of the birth parents. However, the obligation to pay support must be apportioned fairly between the birth parents and other persons who are liable to pay support. In some cases, where there are several persons obligated to pay child support and the circumstances otherwise warrant it, the Court may consider a stepparent's support obligations to be discharged.

Certainly on post-Chartier applications for child support against a step-parent, it is not sufficient for the
Applicant to simply assert marriage or a period of cohabitation. The onus is on the applicant to establish that
The step-parent stood in the place of a parent by addressing all of the factors listed in Chartier. / Bruvels
v. Guindon, [2000] O.J. No. 875 (S.C.J.)./

Some judges tend to view step-parents who contest the imposition of long-term support
Obligations as “good Samaritans” who owe no further duty; others view them, in
Particular step-fathers, as part of the larger social problem of “deadbeat” dads, men who
Are trying to avoid their legitimate social (and especially their financial) obligations to
Their children./Supreme Ct. Law Rev. (2d) 433./