diff --git a/smb/autotests/smburltest.cpp b/smb/autotests/smburltest.cpp
--- a/smb/autotests/smburltest.cpp
+++ b/smb/autotests/smburltest.cpp
@@ -89,6 +89,41 @@
         QCOMPARE(SMBUrl(QUrl("smb://[fe80::9cd7:32c7:faeb:f23d]/share")).toSmbcUrl(),
                  "smb://fe80--9cd7-32c7-faeb-f23d.ipv6-literal.net/share");
     }
+
+    void testWorkgroupWithSpaces()
+    {
+        // Workgroups can have spaces but QUrls cannot, so we have a hack
+        // that puts the workgroup info into a query.
+        // Only applicable to SMB1 pretty much, we do not do workgroup browsing
+        // for 2+.
+        // https://bugs.kde.org/show_bug.cgi?id=204423
+
+        // wg
+        QCOMPARE(SMBUrl(QUrl("smb://?kio-workgroup=hax max")).toSmbcUrl(),
+                 "smb://hax max/");
+        // wg and query
+        QCOMPARE(SMBUrl(QUrl("smb://?kio-workgroup=hax max&q=a")).toSmbcUrl(),
+                 "smb://hax max/?q=a");
+        // host and wg and query
+        QCOMPARE(SMBUrl(QUrl("smb://host/?kio-workgroup=hax max&q=a")).toSmbcUrl(),
+                 "smb://hax max/host?q=a");
+        // host and wg and query
+        QCOMPARE(SMBUrl(QUrl("smb://host/share?kio-workgroup=hax max")).toSmbcUrl(),
+                 "smb://hax max/host/share");
+        // Non-empty path. libsmbc hates unclean paths
+        QCOMPARE(SMBUrl(QUrl("smb:///////?kio-workgroup=hax max")).toSmbcUrl(),
+                 "smb://hax max/");
+        // % character - run through .url() to simulate behavior of our listDir()
+        QCOMPARE(SMBUrl(QUrl(QUrl("smb://?kio-workgroup=HAX%25MAX").url())).toSmbcUrl(),
+                 "smb://HAX%25MAX/");
+        // !ascii - run through .url() to simulate behavior of our listDir()
+        QCOMPARE(SMBUrl(QUrl(QUrl("smb:///?kio-workgroup=DOMÄNE A").url())).toSmbcUrl(),
+                 "smb://DOMÄNE A/"); // works as-is with smbc.
+
+        // Also make sure type detection knows about this
+        QCOMPARE(SMBUrl(QUrl("smb:/?kio-workgroup=hax max")).getType(),
+                 SMBURLTYPE_WORKGROUP_OR_SERVER);
+    }
 };
 
 QTEST_GUILESS_MAIN(SMBUrlTest)
diff --git a/smb/kio_smb_browse.cpp b/smb/kio_smb_browse.cpp
--- a/smb/kio_smb_browse.cpp
+++ b/smb/kio_smb_browse.cpp
@@ -40,8 +40,8 @@
 #include <KLocalizedString>
 
 #include <QEventLoop>
-
 #include <QTimer>
+#include <QUrlQuery>
 
 #include <grp.h>
 #include <pwd.h>
@@ -519,6 +519,16 @@
                 // QString workgroup = m_current_url.host().toUpper();
                 QUrl u("smb://");
                 u.setHost(dirpName);
+                if (!u.isValid()) {
+                    // In the event that the workgroup contains bad characters, put it in a query instead.
+                    // This is transparently handled by SMBUrl when we get this as input again.
+                    // Also see documentation there.
+                    // https://bugs.kde.org/show_bug.cgi?id=204423
+                    u.setHost(QString());
+                    QUrlQuery q;
+                    q.addQueryItem("kio-workgroup", dirpName);
+                    u.setQuery(q);
+                }
                 udsentry.fastInsert(KIO::UDSEntry::UDS_URL, u.url());
 
                 // Call base class to list entry
diff --git a/smb/smburl.cpp b/smb/smburl.cpp
--- a/smb/smburl.cpp
+++ b/smb/smburl.cpp
@@ -34,6 +34,7 @@
 
 #include <QDir>
 #include <QHostAddress>
+#include <QUrlQuery>
 #include <KConfig>
 #include <KIO/Global>
 
@@ -112,10 +113,62 @@
         break;
     }
 
-    if (sambaUrl.url() == "smb:/")
+    // NetBios workgroup names may contain characters that QUrl will not
+    // allow in a host. Yet the SMB URI requires us to have the workgroup
+    // in the host field when browsing a workgroup.
+    // As a hacky workaround we'll not set a host but use a query param
+    // when encountering a workgroup that causes QUrl to error out.
+    // For libsmbc we then need to translate the query back to SMB URI.
+    // Since this is super daft string construction it will doubltlessly
+    // be imperfect and so we do still prefer deferring the string
+    // construction to QUrl whenever possible.
+    // https://support.microsoft.com/en-gb/help/909264/naming-conventions-in-active-directory-for-computers-domains-sites-and
+    // https://bugs.kde.org/show_bug.cgi?id=204423
+    //
+    // Should we ever stop supporting workgroup browsing this entire
+    // hack can be removed.
+    QUrlQuery query(sambaUrl);
+    const QString workgroup = query.queryItemValue("kio-workgroup");
+    if (workgroup.isEmpty()) {
+        // If we don't have a hack to apply we can simply defer to QUrl
+        if (sambaUrl.url() == "smb:/") {
+            m_surl = "smb://";
+        } else {
+            m_surl = sambaUrl.toString(QUrl::PrettyDecoded).toUtf8();
+        }
+    } else {
+        // If we have a workgroup hack to apply we need to manually construct
+        // the stringy URI.
+        query.removeQueryItem("kio-workgroup");
+        sambaUrl.setQuery(query);
+
         m_surl = "smb://";
-    else
-        m_surl = sambaUrl.toString(QUrl::PrettyDecoded).toUtf8();
+        if (!sambaUrl.userInfo().isEmpty()) {
+            m_surl += sambaUrl.userInfo() + "@";
+        }
+        m_surl += workgroup;
+        // Workgroups can have ports per the IANA definition of smb.
+        if (sambaUrl.port() != -1) {
+            m_surl += ':' + QString::number(sambaUrl.port());
+        }
+
+        // Make sure to only use clear paths. libsmbc is allergic to excess slashes.
+        QString path('/');
+        if (!sambaUrl.host().isEmpty()) {
+            path += sambaUrl.host();
+        }
+        if (!sambaUrl.path().isEmpty()) {
+            path += sambaUrl.path();
+        }
+        m_surl += QDir::cleanPath(path);
+
+        if (!sambaUrl.query().isEmpty()) {
+            m_surl += '?' + sambaUrl.query();
+        }
+        if (!sambaUrl.fragment().isEmpty()) {
+            m_surl += '#' + sambaUrl.fragment();
+        }
+    }
 
     m_type = SMBURLTYPE_UNKNOWN;
     // update m_type
@@ -133,7 +186,7 @@
     }
 
     if (path().isEmpty() || path(QUrl::FullyDecoded) == "/") {
-        if (host().isEmpty())
+        if (host().isEmpty() && !query().contains("kio-workgroup"))
             m_type = SMBURLTYPE_ENTIRE_NETWORK;
         else
             m_type = SMBURLTYPE_WORKGROUP_OR_SERVER;