diff --git a/src/cmd/go/internal/modget/get.go b/src/cmd/go/internal/modget/get.go
index 563f1a988f0..8eee723f89b 100644
--- a/src/cmd/go/internal/modget/get.go
+++ b/src/cmd/go/internal/modget/get.go
@@ -38,6 +38,7 @@ import (
 	"cmd/go/internal/base"
 	"cmd/go/internal/imports"
 	"cmd/go/internal/load"
+	"cmd/go/internal/modfetch"
 	"cmd/go/internal/modload"
 	"cmd/go/internal/par"
 	"cmd/go/internal/search"
@@ -1466,6 +1467,8 @@ func (r *resolver) chooseArbitrarily(cs pathSet) (isPackage bool, m module.Versi
 // checkPackageProblems reloads packages for the given patterns and reports
 // missing and ambiguous package errors. It also reports retractions and
 // deprecations for resolved modules and modules needed to build named packages.
+// It also adds a sum for each updated module in the build list if we had one
+// before and didn't get one while loading packages.
 //
 // We skip missing-package errors earlier in the process, since we want to
 // resolve pathSets ourselves, but at that point, we don't have enough context
@@ -1593,9 +1596,52 @@ func (r *resolver) checkPackageProblems(ctx context.Context, pkgPatterns []strin
 		})
 	}
 
+	// Load sums for updated modules that had sums before. When we update a
+	// module, we may update another module in the build list that provides a
+	// package in 'all' that wasn't loaded as part of this 'go get' command.
+	// If we don't add a sum for that module, builds may fail later.
+	// Note that an incidentally updated package could still import packages
+	// from unknown modules or from modules in the build list that we didn't
+	// need previously. We can't handle that case without loading 'all'.
+	sumErrs := make([]error, len(r.buildList))
+	for i := range r.buildList {
+		i := i
+		m := r.buildList[i]
+		mActual := m
+		if mRepl := modload.Replacement(m); mRepl.Path != "" {
+			mActual = mRepl
+		}
+		old := module.Version{Path: m.Path, Version: r.initialVersion[m.Path]}
+		if old.Version == "" {
+			continue
+		}
+		oldActual := old
+		if oldRepl := modload.Replacement(old); oldRepl.Path != "" {
+			oldActual = oldRepl
+		}
+		if mActual == oldActual || mActual.Version == "" || !modfetch.HaveSum(oldActual) {
+			continue
+		}
+		r.work.Add(func() {
+			if _, err := modfetch.DownloadZip(ctx, mActual); err != nil {
+				verb := "upgraded"
+				if semver.Compare(m.Version, old.Version) < 0 {
+					verb = "downgraded"
+				}
+				replaced := ""
+				if mActual != m {
+					replaced = fmt.Sprintf(" (replaced by %s)", mActual)
+				}
+				err = fmt.Errorf("%s %s %s => %s%s: error finding sum for %s: %v", verb, m.Path, old.Version, m.Version, replaced, mActual, err)
+				sumErrs[i] = err
+			}
+		})
+	}
+
 	<-r.work.Idle()
 
-	// Report deprecations, then retractions.
+	// Report deprecations, then retractions, then errors fetching sums.
+	// Only errors fetching sums are hard errors.
 	for _, mm := range deprecations {
 		if mm.message != "" {
 			fmt.Fprintf(os.Stderr, "go: module %s is deprecated: %s\n", mm.m.Path, mm.message)
@@ -1615,6 +1661,12 @@ func (r *resolver) checkPackageProblems(ctx context.Context, pkgPatterns []strin
 	if retractPath != "" {
 		fmt.Fprintf(os.Stderr, "go: to switch to the latest unretracted version, run:\n\tgo get %s@latest\n", retractPath)
 	}
+	for _, err := range sumErrs {
+		if err != nil {
+			base.Errorf("go: %v", err)
+		}
+	}
+	base.ExitIfErrors()
 }
 
 // reportChanges logs version changes to os.Stderr.
diff --git a/src/cmd/go/testdata/script/mod_get_update_unrelated_sum.txt b/src/cmd/go/testdata/script/mod_get_update_unrelated_sum.txt
new file mode 100644
index 00000000000..0093c0eda0c
--- /dev/null
+++ b/src/cmd/go/testdata/script/mod_get_update_unrelated_sum.txt
@@ -0,0 +1,120 @@
+# Check that 'go get' adds sums for updated modules if we had sums before,
+# even if we didn't load packages from them.
+# Verifies #44129.
+
+env fmt='{{.ImportPath}}: {{if .Error}}{{.Error.Err}}{{else}}ok{{end}}'
+
+# Control case: before upgrading, we have the sums we need.
+# go list -deps -e -f $fmt .
+# stdout '^rsc.io/quote: ok$'
+# ! stdout rsc.io/sampler  # not imported by quote in this version
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+go mod tidy
+cmp go.mod.orig go.mod
+cmp go.sum.orig go.sum
+
+
+# Upgrade a module. This also upgrades rsc.io/quote, and though we didn't load
+# a package from it, we had the sum for its old version, so we need the
+# sum for the new version, too.
+go get -d example.com/upgrade@v0.0.2
+grep '^rsc.io/quote v1.5.2 ' go.sum
+
+# The upgrade still breaks the build because the new version of quote imports
+# rsc.io/sampler, and we don't have its zip sum.
+go list -deps -e -f $fmt
+stdout 'rsc.io/quote: ok'
+stdout 'rsc.io/sampler: missing go.sum entry for module providing package rsc.io/sampler'
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Replace the old version with a directory before upgrading.
+# We didn't need a sum for it before (even though we had one), so we won't
+# fetch a new sum.
+go mod edit -replace rsc.io/quote@v1.0.0=./dummy
+go get -d example.com/upgrade@v0.0.2
+! grep '^rsc.io/quote v1.5.2 ' go.sum
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Replace the new version with a directory before upgrading.
+# We can't get a sum for a directory.
+go mod edit -replace rsc.io/quote@v1.5.2=./dummy
+go get -d example.com/upgrade@v0.0.2
+! grep '^rsc.io/quote v1.5.2 ' go.sum
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Replace the new version with a different version.
+# We should get a sum for that version.
+go mod edit -replace rsc.io/quote@v1.5.2=rsc.io/quote@v1.5.1
+go get -d example.com/upgrade@v0.0.2
+! grep '^rsc.io/quote v1.5.2 ' go.sum
+grep '^rsc.io/quote v1.5.1 ' go.sum
+cp go.mod.orig go.mod
+cp go.sum.orig go.sum
+
+
+# Delete the new version's zip (but not mod) from the cache and go offline.
+# 'go get' should fail when fetching the zip.
+rm $GOPATH/pkg/mod/cache/download/rsc.io/quote/@v/v1.5.2.zip
+env GOPROXY=off
+! go get -d example.com/upgrade@v0.0.2
+stderr '^go: upgraded rsc.io/quote v1.0.0 => v1.5.2: error finding sum for rsc.io/quote@v1.5.2: module lookup disabled by GOPROXY=off$'
+
+-- go.mod.orig --
+module m
+
+go 1.16
+
+require (
+	example.com/upgrade v0.0.1
+	rsc.io/quote v1.0.0
+)
+
+replace (
+	example.com/upgrade v0.0.1 => ./upgrade1
+	example.com/upgrade v0.0.2 => ./upgrade2
+)
+-- go.sum.orig --
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+-- go.sum.want --
+golang.org/x/text v0.0.0-20170915032832-14c0d48ead0c/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+rsc.io/quote v1.0.0 h1:kQ3IZQzPTiDJxSZI98YaWgxFEhlNdYASHvh+MplbViw=
+rsc.io/quote v1.0.0/go.mod h1:v83Ri/njykPcgJltBc/gEkJTmjTsNgtO1Y7vyIK1CQA=
+rsc.io/quote v1.5.2 h1:3fEykkD9k7lYzXqCYrwGAf7iNhbk4yCjHmKBN9td4L0=
+rsc.io/quote v1.5.2/go.mod h1:LzX7hefJvL54yjefDEDHNONDjII0t9xZLPXsUe+TKr0=
+rsc.io/sampler v1.3.0/go.mod h1:T1hPZKmBbMNahiBKFy5HrXp6adAjACjK9JXDnKaTXpA=
+-- use.go --
+package use
+
+import (
+	_ "example.com/upgrade"
+	_ "rsc.io/quote"
+)
+-- upgrade1/go.mod --
+module example.com/upgrade
+
+go 1.16
+-- upgrade1/upgrade.go --
+package upgrade
+-- upgrade2/go.mod --
+module example.com/upgrade
+
+go 1.16
+
+require rsc.io/quote v1.5.2 // indirect
+-- upgrade2/upgrade.go --
+package upgrade
+-- dummy/go.mod --
+module rsc.io/quote
+
+go 1.16
+-- dummy/quote.go --
+package quote
+