I'm using Sharepoint online through Office 365 and am connected via Sharepoint Designer. I'm attempting to learn how to customize the master page however I cannot find Seattle.html when I go to All Files > _catalogs > masterpage

Any ideas what I'm doing wrong?

Thanks in advance!

What type of site did you create?

All master Pages are found under that specific folder and is automatically detected inside SharePoint

Or can also be found under Master Page quick links on the left

Note: If you still can't find the master page, try creating a new site. Also it is recommended not to modify the original seattle and oslo masterpage. Try make a copy of it for all modifications.

Hi,

Per my knowledge you cannot use SharePoint designer to edit the html file associated with the master page. Its locked for editing in SharePoint Designer.

Use Design manager to edit/Customize the master page. 

Design Manager

Hi

For your issue, navigate to the site, Site Settings->Master pages and page layouts:

Do seattle.html and seattle.master exist in the Master Page Gallery?

If not, create a new site collection and copy the seattle.html from it, then upload it to current site's Master Page Gallery and publish it as a major version.

Then open the site with SharePoint Designer again, check whether you can see seattle.html and seattle.master in SharePoint Designer.

Best Regards

Lisa Chen

Thank you for all of your suggestions.

It turns out I can access seattle.html through Office 365 via SharePoint Designer only after activating the SharePoint Server Publishing Infrastructure.

Here are the instructions for doing that for those that may have this issue:

To active the feature, follow these steps:
1. Go to Site settings on the top level site.
2. Under Site Collection Administration, click Site collection features.
3. Find SharePoint Server Publishing Infrastructure, click activate.
 
You should now be able to go back to site settings and access design manager under look and feel or use SharePoint Designer where you will now be able to access the Master Pages quick link as well as locate and edit Seattle.html via All Files > _catalogs > masterpage

Hope this helps!

• Proposed as answer by Ouzayr Khedun 2 hours 36 minutes ago

Yes I completely forgot that. Publishing feature gives you access to master page and other editing features