The Scrum Guide is explicit that the Scrum Master serves the Product Owner, not just the Developers, but this part of the accountability tends to get far less attention in both training and practice than the Scrum-Master-to-team relationship. It's worth being concrete about what this support actually looks like.
Helping find effective techniques for Product Backlog management. Many Product Owners arrive in the role without formal training in backlog management specifically: they may have deep domain or business knowledge without having developed skills in ordering, sizing, or structuring a backlog for effective planning. A Scrum Master who can genuinely coach on techniques like effective backlog ordering, splitting oversized items, or writing clearer acceptance criteria is providing real, concrete support, not just moral encouragement.
Helping the Scrum Team understand the need for clear, concise Product Backlog items. This is a two-way translation function: helping the Product Owner understand what level of detail the Developers actually need to plan and estimate well, and helping the Developers understand why certain ambiguity in early-stage items is normal and not a failure of the Product Owner's diligence.
Facilitating Scrum events as requested or needed. Product Owners sometimes need direct facilitation help, particularly for Sprint Reviews with a complicated stakeholder mix, or for backlog refinement sessions where competing priorities need careful navigation.
Coaching on empirical product planning in a complex environment. Product Owners are frequently under pressure from stakeholders to commit to fixed roadmaps far in advance, in ways that conflict with the empirical, adapt-as-you-learn approach Scrum is built on. A Scrum Master who helps the Product Owner navigate this tension (including sometimes helping them push back on stakeholder pressure for false certainty) is providing a genuinely valuable and often underappreciated form of support.
Protecting the Product Owner from having their role hollowed out. In some organizations, pressure builds for a committee, a manager, or a group of stakeholders to effectively make backlog decisions, leaving the named Product Owner as a figurehead without real authority. A Scrum Master who notices this drift and actively works to preserve genuine Product Owner authority, rather than letting the accountability quietly become nominal, is protecting one of the structural conditions Scrum actually depends on to function.
The common theme across all of this: Scrum Master support for the Product Owner isn't primarily about making their life easier in a generic sense. It's about protecting the specific conditions (real backlog authority, workable planning techniques, a defensible position against unreasonable stakeholder pressure) that let the Product Owner accountability function the way the framework assumes it will.