diff --git a/rslib/src/tags.rs b/rslib/src/tags.rs
index 977649f0c..2a0355f14 100644
--- a/rslib/src/tags.rs
+++ b/rslib/src/tags.rs
@@ -12,11 +12,7 @@ use crate::{
 
 use regex::{NoExpand, Regex, Replacer};
 use std::cmp::Ordering;
-use std::{
-    borrow::Cow,
-    collections::{HashMap, HashSet},
-    iter::Peekable,
-};
+use std::{borrow::Cow, collections::HashSet, iter::Peekable};
 use unicase::UniCase;
 
 #[derive(Debug, Clone)]
@@ -120,30 +116,50 @@ fn normalize_tag_name(name: &str) -> String {
     out.trim_end_matches("::").into()
 }
 
-fn fill_missing_tags(tags: Vec<Tag>) -> Vec<Tag> {
-    let mut filled_tags: HashMap<String, Tag> = HashMap::with_capacity(tags.len());
-    for tag in tags.into_iter() {
-        let name = tag.name.to_owned();
-        let split: Vec<&str> = (&tag.name).split("::").collect();
-        for i in 0..split.len() - 1 {
-            let comp = split[0..i + 1].join("::");
-            let t = Tag::new(comp.to_owned(), Usn(-1));
-            if filled_tags.get(&comp).is_none() {
-                filled_tags.insert(comp, t);
-            }
-        }
-        if filled_tags.get(&name).is_none() {
-            filled_tags.insert(name, tag);
-        }
-    }
-    let mut tags: Vec<Tag> = filled_tags.values().map(|t| (*t).clone()).collect();
-    tags.sort_unstable();
-
-    tags
+fn immediate_parent_name(tag_name: UniCase<&str>) -> Option<UniCase<&str>> {
+    tag_name.rsplitn(2, '\x1f').nth(1).map(UniCase::new)
 }
 
-fn tags_to_tree(tags: Vec<Tag>) -> TagTreeNode {
-    let tags = fill_missing_tags(tags);
+/// For the given tag, check if immediate parent exists. If so, add
+/// tag and return.
+/// If the immediate parent is missing, check and add any missing parents.
+/// This should ensure that if an immediate parent is found, all ancestors
+/// are guaranteed to already exist.
+fn add_self_and_missing_parents<'a, 'b>(
+    all: &'a mut HashSet<UniCase<&'b str>>,
+    missing: &'a mut Vec<UniCase<&'b str>>,
+    tag_name: UniCase<&'b str>,
+) {
+    if let Some(parent) = immediate_parent_name(tag_name) {
+        if !all.contains(&parent) {
+            missing.push(parent.clone());
+            add_self_and_missing_parents(all, missing, parent);
+        }
+    }
+    // finally, add self
+    all.insert(tag_name);
+}
+
+/// Append any missing parents. Caller must sort afterwards.
+fn add_missing_parents(tags: &mut Vec<Tag>) {
+    let mut all_names: HashSet<UniCase<&str>> = HashSet::new();
+    let mut missing = vec![];
+    for tag in &*tags {
+        add_self_and_missing_parents(&mut all_names, &mut missing, UniCase::new(&tag.name))
+    }
+    let mut missing: Vec<_> = missing
+        .into_iter()
+        .map(|n| Tag::new(n.to_string(), Usn(0)))
+        .collect();
+    tags.append(&mut missing);
+}
+
+fn tags_to_tree(mut tags: Vec<Tag>) -> TagTreeNode {
+    for tag in &mut tags {
+        tag.name = tag.name.replace("::", "\x1f");
+    }
+    add_missing_parents(&mut tags);
+    tags.sort_unstable_by(|a, b| UniCase::new(&a.name).cmp(&UniCase::new(&b.name)));
     let mut top = TagTreeNode::default();
     let mut it = tags.into_iter().peekable();
     add_child_nodes(&mut it, &mut top);
@@ -153,7 +169,7 @@ fn tags_to_tree(tags: Vec<Tag>) -> TagTreeNode {
 
 fn add_child_nodes(tags: &mut Peekable<impl Iterator<Item = Tag>>, parent: &mut TagTreeNode) {
     while let Some(tag) = tags.peek() {
-        let split_name: Vec<_> = tag.name.split("::").collect();
+        let split_name: Vec<_> = tag.name.split('\x1f').collect();
         match split_name.len() as u32 {
             l if l <= parent.level => {
                 // next item is at a higher level
@@ -477,8 +493,8 @@ mod test {
         let mut col = open_test_collection();
         let nt = col.get_notetype_by_name("Basic")?.unwrap();
         let mut note = nt.new_note();
-        note.tags.push("foo::a".into());
-        note.tags.push("foo::b".into());
+        note.tags.push("foo::bar::a".into());
+        note.tags.push("foo::bar::b".into());
         col.add_note(&mut note, DeckID(1))?;
 
         // missing parents are added
@@ -487,21 +503,64 @@ mod test {
             node(
                 "",
                 0,
-                vec![node("foo", 1, vec![leaf("a", 2), leaf("b", 2)])]
+                vec![node(
+                    "foo",
+                    1,
+                    vec![node("bar", 2, vec![leaf("a", 3), leaf("b", 3)])]
+                )]
             )
         );
 
         // differing case should result in only one parent case being added -
         // the first one
-        *(&mut note.tags[1]) = "FOO::b".into();
         col.storage.clear_tags()?;
+        *(&mut note.tags[0]) = "foo::BAR::a".into();
+        *(&mut note.tags[1]) = "FOO::bar::b".into();
         col.update_note(&mut note)?;
         assert_eq!(
             col.tag_tree()?,
             node(
                 "",
                 0,
-                vec![node("foo", 1, vec![leaf("a", 2), leaf("b", 2)])]
+                vec![node(
+                    "foo",
+                    1,
+                    vec![node("BAR", 2, vec![leaf("a", 3), leaf("b", 3)])]
+                )]
+            )
+        );
+
+        // things should work even if the immediate parent is not missing
+        col.storage.clear_tags()?;
+        *(&mut note.tags[0]) = "foo::bar::baz".into();
+        *(&mut note.tags[1]) = "foo::bar::baz::quux".into();
+        col.update_note(&mut note)?;
+        assert_eq!(
+            col.tag_tree()?,
+            node(
+                "",
+                0,
+                vec![node(
+                    "foo",
+                    1,
+                    vec![node("bar", 2, vec![node("baz", 3, vec![leaf("quux", 4)])])]
+                )]
+            )
+        );
+
+        // numbers have a smaller ascii number than ':', so a naive sort on
+        // '::' would result in one::two being nested under one1.
+        col.storage.clear_tags()?;
+        *(&mut note.tags[0]) = "one".into();
+        *(&mut note.tags[1]) = "one1".into();
+        note.tags.push("one::two".into());
+        col.update_note(&mut note)?;
+        assert_eq!(
+            col.tag_tree()?,
+            node(
+                "",
+                0,
+                vec![node("one", 1, vec![leaf("two", 2)]), leaf("one1", 1)]
             )
         );